These are great little units that you can use to have your hard drives in and remove them from your PC without having to unscrew everything to open it up. The little draws just side in and out with your hard drive inside. I don't know why more people don't use them.
For security, you can take out the hard drive and keep it somewhere safe, or use it in a different PC.
Its useful to be able to put new operating system on your PC, without having to 'dual boot.'
I have got the new Windows 7 on one and Ubuntu 9.04 the J.J. on another. To boot up and be on the internet, Windows 7 takes 2 mins 20 seconds, Ubuntu takes 1 min 15 seconds.
Check when in use that the hard drive is well ventilated and doesn't get too hot, or you may need an extra fan.
Also, if you are going to be changing the hard drive inside the caddy often, it may be worth having more than one, as after repeated use the 40 way IDE cable can eventually fail.