I use a Merc Stealth myself and love it. It basically puts a separate game pad on the left side of the keyboard, with common buttons all nicely laid out around the "wasd". I don't even fully use the configuration software that came with it, I just use the in-game key bindings, but it does come with preset and customizable configs for pretty much any game you can think of. Honestly I can't imagine playing any other way now.
It also has a slick glow, which can be cycled through blue, red, and purple if you're into that sort of thing. It's actually very nice if I'm playing with the lights out.