there are alternatives, there is the m-audio trigger and the akai MPD.
the mpd looks like its got good weight to it, wereas the trigger looks a little plasticy.
i never used one but it seems like a waste of space to me.
for what it does i mean
TMD are gonna start a 4deck/MPC/KAOS PAD extravaganza at some point this year to do as a "live" show.... need to get my hands on a MPC... pretty expensive though, so ive been looking at a few cheaper alternatives.... but non quite match up really..
you know whats bad about this... its cant be standalone.... MPCs you can use without it being attached to a laptop/pc... which is a hell of alot better when using it for other purposes... and on the fly beat making...