I concur, DMT diamond stones are excellent. Perfect for a quick touch up and they work well with even the hardest of modern blade steels.
I prefer this model though:
http://theconsumerlink.com/DIAMONDMACHININGTECHNOLOGY/detail/TCL+FWFC/110 It's a bit bigger than the keychain size model, but it's faster to use and you get both a course and fine edge.