Every PC I set up for business has one for backing up - either mounted internally as usual or used as an external USB enclosure with a USB to IDE cable kit. Expensive ones usually don't work any better than cheaper ones.
The CRU DataPort brand is the original and still a good choice sold at many places like Provantage.com and CDW.com.
http://www.computergate.com has a good selection of inexpensive ones. Kingwin and Athena make some of the better looking ones.
http://www.dirtcheapdrives.com carry some brands that aren't often seen elsewhere but their prices are steep.
What I like to see is an enclosed frame, as if the mobile rack is not enclosed in some way (perhaps partially accomplished if an open rack was sandwiched tightly by optical drives above and below) the built-in fan(s) can't be effective. IOW you want air to be flowing around the drive. Air, like lots of other things, takes the path of least resistance. And a switch that cuts the power to the drive as well as locking the mobile part in place.
.bh.