Used to use them for backup onto old HDD's, until I set up my file server and tape backup.
Basically they are pretty much the same. Get one with it's own internal fan though, unless you ant to cook the indie of your PC some more.
Bear in mind the cheap IDE ones aren't
hot swappable, unless the HDD's you use are
exactly the same (not just the same size - must be exact same model too). The reason is that they still have to be detected in BIOS on boot, and will throw your OS into a hissy fit if you try to swap one that's different.

In other words, you have to power down, swap the drive, then reboot., to change HDD's.
So, with those provisos, they can be useful if you want extra storage or something, and have some old spare HDD's.
Or, if ya want to caary your full OS between machines, and have a rack in each PC.
Cheers
Mick