You can assign whatever samples you want to whatever pad/pedal you want, and even hook up 2 drum kits to have more 'drums'. You can just jam out or record yourself for later playback.
You can find the program here at his website. There are two versions, 1.19 which has the basic drum kit samples, and 1.25 is the newer one, slightly larger download but with more drum samples and support for using 2 drum sets at once.
You will need Direct X 9.0c, the Microsoft Xbox360 controller drivers for Windows XP or Vista (or the Sony PS3 controller drivers if you're going that route >_>, and at least 128mb of RAM.
There is also a small (750kb or so) program called DMR2WAV that will let you take files output by the program and convert them to .wav, .mp3, and more. The program is hosted on the same page/site.
The RB drums make for a pretty good practice or jam device, but hitting them too hard can trigger multiple hits, and playing them too fast can cause missed notes. There are config settings in the Drum Machine software that can help with this. Still, not bad for an $80 'electronic drum kit'
+edit+ uh, the site is down right this second but should be back up shortly.