We've played one session of my new C&C campaign set in my old AD&D campaign world. I'm using Keep on the Borderlands, and it was great! We have a wholly new player (fiance of an old regular) so there was a bit more explanation than usual, but the session when quite fast and smoothly. I used be a very "prepared" referee, needing ample time to "get everything just right" for a session but my experience combined with the easy to use C&C mechanics made winging it a breeze. I used my Moldvay D&D Basic book as a companion to the C&C Collector's Boxed Set. Works great!