Never understood why anyone in their right mind would buy those things. You've already got a device in front of you with over 100 keys on it that does just as good/an even better job. Spend your money on something more useful.
Well I'll tell you why for some people they do a better job....
Wireless keyboards.
Ever tried to game on one of those? For those of us who like wireless for day to day stuff, and that don't want to have two keyboards sitting on their desk, these things are perfect. I've got the Belkin one.