I have a Nektar Impact LX25+ and it worked in Reaper with very little effort. I think I might have gone on their site though and downloaded something for mapping controls within Reaper though, but if so, it was a very easy process. I really recommend their keyboards though. I had an older model M-audio Axiom 49 before, but I sold that, and ended up buying the Nektar, and I absolutely love it. Perfect size for what I do too, as I can tuck it under my monitor stand when not in use. The value for the price is also very surprising. Great keyboards, but won't break the bank.
Actually here is what I followed to set things up in Reaper (from Nektar's website):
http://support.nektartech.com/s.nl/c....f?category=26