I just bought one last weekend. It is essentially a midi-controller that you hook up to a VSTi in Reaper (I use mostly Battery). You can effectively do the same with a MIDI keyboard (I did that before) but with the square pads you have a much better feel. They also have a better response to 'tapping' then a keyboard.
If you don't play/program your own beats then a Padcontrol will be close to useless I think. But as it is 'just' another controller, you can hook it up to anything, like triggering chords and samples and stuff
It also has a X-Y touchpad and I have a few VSTi synths that can use that for modulation (still have to try it out though).