Have had no experience with these but they seem to be all the rage, just good for extra storage???
and is usb preferable to what ever othe rmeans of hooking it up??

Good for extra storage.

Also for data backups. (You can disconnect them easily for safe storage... so when a power spike fries you internal disks... you'll still have a backup)

And for transport. You can easily hook'm up to a different computer.

And for file-sharing in your home LAN.
(You can get 'm including an OS, and you can easily set 'm up to share the new storage space in your network)

When using USB, you need some patience.

Nowadays, me thinks there's also external Sata ones. Probably much faster than USB ones.
Whether those are hot-pluggable, I dunno...

If you only need it for storage, only get an external disk if for whatever reason you cannot install an internal disk.

I've got two 1TB external drives (I've had very bad luck with drive failures in the past), both of which have both USB and eSATA connections. Firewire is another popular interface. eSATA is indeed subjectively quicker (I haven't run any performance tests), though depending on your motherboard and the drive, there may be problems if you reboot with the drives connected and powered up. Even setting the boot priorities in BIOS, my boot sequence would hang with one of the drives connected, but not the other. The simple solution for me was to go to USB for that drive, but leaving it powered off when not running a backup (and connected via eSATA) should work, too. Neither of these are the "one touch" backup solutions of the Maxtor or Seagate variety, since those are fairly simple, idiot-proof, with less ability for me to program backups and selectively restore files.

I'm still looking for decent backup software to complement these drives though. I can't recommend Norton Ghost. Using this, it would run for eight hours, then fail, with nothing written to the backup, and only a generic "an error has occured" message in the log. Tech support is slow to respond, outsourced, and in general unable to solve technical issues (Norton sent me the "how are we doing?" questionnare before tech support had even responded in their 96 hour!?! window. I think Symantec needs to move a few paychecks from marketing to tech support and software development.)

Windows XP has a rudimentary backup utility, but it is slow, and writes to a proprietary format that you can't selectively browse. I'm tentitively leaning towards either Shadowprotect or Acronis True Image, but I've still got to do a little more reading to avoid repeating the nasty experience of Norton Ghost.