Here is a complete and detailed review for those of you who want to know the strengths and weaknesses of Japan as well as whether the DLC is worth the price.
Pillbox (Extremely Resistant Bunker) $25 – Average but poor range anti-tank and anti-infantry. Not very useful but a very tough early-game defensive position.
105mm Position (Fragile Bunker) $30 – Very expensive considering one shot will level it. If your enemy is only using tanks it can help slightly but light artillery or even fighter-bombers will take this out. Deadly gun but no very useful.
AA Position (Fragile Bunker) $20 – Nigh on useless it is poor for killing planes or infantry and so easily destroyed that it is generally not the best option at all.
Barracks $15 - This is a very useful building for the Japanese as it contains some very good units for good prices as well as being incredibly cheap in itself. While not useful for killing anything coming its way the Barracks does provide some very useful and perhaps necessary units.
Kurogane (Recon) $5 – Very cheap, like the Willys jeep and in the same league. Very good recon range (suspiciously so) and so extraordinarily good value for money. Its only disadvantage is that it can be a bit slow to get to the front line.
Konoe (Light Infantry) $5 – Cheap and basic these are mostly useful for capturing buildings rather than actual combat due to their weak weapons. They can take most basic infantry but will be easily outclassed by more advanced units.
Sogekihei (Sharpshooters) $15 ($25 research) – Expensive individually but deadly against infantry or towed units. Very slow rate of fire limits them and despite an initial surprise they will be quickly massacred by artillery fire.
Dokuritsu Kohei (Combat Engineers) $10 ($25 research) – Very good value as with their flamethrowers they can take most other infantry and can quickly destroy buildings. Although they cannot capture they are still a force to be reckoned with. Limited range can handicap them however.
Armour Base $35 – This is quite expensive anyway without considering what you get inside. Some good units can be found here but as an early game investment it can be quite poor.
Ha-Go (Light Tank) $5 – No armour mean that it is incredibly vulnerable to any attack and a weak gun means that it can be a pointless investment. However, in a large swarm these can take out super-heavy tanks with minimal losses so are actually quite deadly if used well.
Chi-Ha (Medium Tank) $20 – Fairly weak in all respects but overall not bad. Unfortunately it isn't great either and a fairly high price means its use is limited. 2 armour is sufficient but only just. Useful but not brilliant.
Chi-Nu (Advanced Medium Tank) $25 ($50 research) – Its monstrous research price and minimal improvements make this a late game investment that doesn't provide you with a very good tank. Dangerously fast but underpowered and no more armour mean that it could be far more useful than it actually is. A let-down.
Chi-Ri (Heavy Tank) $30 ($25 research) – Fairly good overall with 3 armour and an average gun with good range but being bolted onto the medium tank upgrade tree destroys its potential. Fairly high price means that it isn't great and it is fairly easily taken by good Tank Destroyers.
Ta-Se (Mobile Anti-Aircraft) $25 ($50 research) – A fairly solid AA gun with 3 armour. Its cheap price means that it can be deployed in good numbers and it can stand up to some punishment. The high research price means that it is for later game but it is one of the best units Japan has.
Artillery and Anti-Air Base $45 – Expensive as a base but contains several fairly useful units. Can be hard to use properly.
Type 88-75mm (AA/AT) $20 – Versatile is the word. Average Anti-Air capacity and useful Anti-Tank capacity means that it is a good unit to deploy but is needed in number. Very weak to artillery but a good initial investment.
Type 35-75mm (Light Artillery) $10 – Useful and cheap. Fairly short (800m) range but no different to others in its class. Powerful and useful but an offensive weapon for an initially defensive faction.
Ku-Se (Armoured Artillery) $20 ($25 research) – Very good value for money but less powerful than its foreign counterparts. This combined with a pitiful 800m range for armoured artillery make this not great although with 2 armour it is better armoured than most other artillery and can withstand an AT shot.
Anti-Tank Base $35 – Sorely needed but can easily fail to provide the anti-tank capacity that Japan so desperately needs.
Type 1-47mm (AT gun) $10 – Poor damage means that it isn't really up to much but with 500m range it can take early tanks. Weak to artillery or concentrated tank assault this is only for early game.
Ho-Ru (Tank Destroyer) $10 – With 2 armour it can stand up to early game punishment but its pitiful range of only 300m and low damage means that it is mainly used for light tank swarms or early-game assault guns. Needs to be quickly replaced but does have limited use.
Ho-Ri (Advanced Tank Destroyer) $35 ($75 research) – Devastating firepower and 5 armour mean that this is a vicious piece of kit. Very useful as a defensive vehicle as it hasn't got a turret. High research price prevents early-game deployment but definitely worth its reasonable price tag. It presents the Japanese with a good anti-tank gun.
Airfield $45 – A very good investment as it is cheap and can provide generally good units. Versatile due to high range of units.
Susie (Recon Plane) $25 – Expensive for recon and unarmed. Not very useful due to the Kurogane recon car.
Giretsu Kuteitai (Recon Paratroopers) $15 – Very good recon range and can be dropped in to cover the whole map. Not a necessary investment for the Japanese but can be useful. Fairly good anti-infantry capacity.
Teishin Shudan (Paratroopers) $10 ($25 research) – Good at killing infantry and cheap for paratroopers. Become the heavy infantry for Japan.
Zero (Fighter) $20 – Cheap and the best basic fighter. Very fast and good value for money but will probably lose out to advanced fighters.
Shinden (Advanced Fighter) $30 ($50 research) – The weirdest looking plane in the game. It flies backwards! A match to the Me262 but slower as it isn't a jet. Brilliant at ground strafing and fast but still expensive. (Outspammable by Spitfires)
Susei (Fighter-Bomber) $30 ($25 research) – Expensive to field but poor at killing tanks so is quite a poor investment. Needed but lets the faction down as too weak and expensive.
Renzan (Heavy Bomber) $35 ($50 research) – Expensive research prevents early use but as deadly as a B-17 (4 for a HQ). Fairly fast for a bomber (360kmh) but it isn't going to be outrunning any fighters. Lethal defensive armament, better than any other plane. The only planes that can take this are Spitfires, Shindens, Me262s, P-51 Mustangs and Veltros. Combined with fanaticism these can probably rout even those. The deadliest bomber in the game but ground AA will be the best defence against these.
Prototype Base $125 – Can be a good investment but only for late game. Very good units but can be expensive.
A.O.V. (Armoured Recon) $20 – With 4 armour this can withstand fairly heavy fire. Very useful but hampered as it cannot enter woodlands. Good recon range.
O-I (Super-heavy Tank) $55 ($25 research) – Only 4 armour means that it can suffer against other super-heavy tanks but a lethal main gun on a turret means that its AT capacity is solid. Any infantry that gets close will be flamethrowered. This can also help in close tank combat. However its pièce de résistance is its “light” rockets that will annihilate unarmoured targets and damage any armour with a deadly 800m range. This can annihilate most things in its path but is vulnerable to super-heavy tanks as stated.
Ha-To (Armoured Artillery) $40 ($25 research) – Cheap research but a high price mean that it is easy to get but not in number. Only 1 armour means that it can be weak but its 1.2km range means that it is a good piece of artillery. Its gun is also an assault gun as good as the Sturmtiger but with a better range so it will annihilate buildings and badly damage even lightly armoured targets.
Overall they can be a weak faction in the early-game with no solid tactic for defence and several solid counters for any of those defensive strategies. Main strengths lie in the barracks, airfield and prototype base. The infantry are deadly but will be easily countered after their initial use. All infantry are only armed with satchel charges which are nearly useless at killing tanks as they do very little damage. Artillery is good but short-ranged and the tanks are generally weakly armoured but well-armed. Planes are strong and a solid investment but aren't on par with the British. With poor anti-tank capacity overall they can find it hard to counter tanks with anything but the latest units (O-I and Ho-Ri)
In 1939 the Japanese have very limited units but they are spammable and some good tactics are available, particularly considering that the dual purpose gun can be bought. The sharpshooters can be useful in sneakily killing artillery coming for your dual purpose guns and the medium tank is alright at both anti-infantry and anti-tank. The only things useful in the airfield however are the Zero if you need to kill fighter-bombers and the recon paratroops.
In 1942 they can actually be worse than in 1939 due to their opponents getting a better variety of units. The anti-tank base finally becomes available but only with the mediocre Ho-Ru and the light AT gun. This coupled with the fact that the Chi-Ri (Advanced Medium) isn't that much better than the normal medium means that in terms of anti-tank combat they are seriously lacking. The fighter-bomber finally arrives and although it isn't brilliant it can get the job done if used properly. The planes now though will start losing out as more advanced fighters and AA take to the scene. Japanese tactics stay fairly similar, relying on the dual purpose gun to keep them safe from tanks and aerial units. The Ta-Se becomes available and helps in AA support though. A strange thing however: In 1942 the armour base and artillery and AA base are $5 cheaper than normal for no apparent reason.
In 1945 they have all of their strong units and are a force to be reckoned with after an initial period of discomfort.
My overall verdict is that they are not overpowered as there are plenty of solid tactics to defeat them. Nor do I think that they overshadow the British who can easily counter fighters with the spammable Spitfire and who have infinitely better fighter-bombers. They are a worthy faction to enter the game and worth the price for the DLC not even counting the operations (which aren't too great to be honest).
EDIT: Advanced Medium Tank price added. Sorry!
EDIT 2: Ho-Ru updated aswell as 1939 and 1942 strengths and weaknesses.