I have two, both contain hard drives. They can be a valuable part of your storage strategy. I plan to use them for multimedia files, or carrying big files to friends' computers here in town. Backing up your operating system could work, but I've never tried it. Units that are below 128/137 GB probably still come as FAT32 but you can reformat to NTFS (remember, Linux can't read/write NTFS very good yet).
I have a CompUSA unit that looks A LOT like ako's Mentor. CompUSA SKU 312100, barcode number 8 41808 00145 1. Insert your own HD, no fan, no firewire (though there is a sister unit w/1394). I can't get USB 2.0 drivers to install for me (98SE), so it's not that great, but other Windows versions don't need drivers for usb HDs, so you might have no trouble at all. Does get warm after a while, sorry to say.
I also have the Acomdata with the CompUSA logo on it (came with 80 GB FAT32 WD Caviar inside). Failed right out of the box, warranty replacement is just fine. Drivers installed great. Not warm, air gap between drive and case.
No fan on either one.