Every drummer ought to have one. I'm currently using Promark's 2-sided 8-inch XPAD.
It's my first practice pad, so I'm not sure if it's a good one. I've had some problems with hurting wrists (mainly with downstrokes) while practising incessantly on it. I'm in the midst of figuring out if it's a technique or pad problem.
I've heard a lot of recommendations for HQ Real Feel pads. I'll probably buy one these days.