http://www.alesis.com/epercussionInfo about Alesis percussion stuff is here^^^. Not a lot about the Perc Pad - just a brochure and a quick start guide.
It's a small, simple, portable, pre-set handy tool. For the price, I wouldn't expect a lot of bells and whistles. But it's pretty cool it has a kick input.
Sort of the modern cowbell for the 21st century.
I wouldn't confuse it with things like the Performance Pad, and Performance Pad Pro which are more programmable. It's my understanding that the Perf. Pad basically has the SR16 inside, and the Perf. Pad Pro has the SR18.
Neither one is a good option for adding extra external triggers to an existing setup though. They work better as standalone units.
Like Hellfire says, the ControlPad, (or Trigger I/O) would be a better option for that.