I recently acquired a mk1 Genesis and Sega CD set with a couple of games for very cheap (along with an original NES, although I'm not sure I will keep that).
A couple of exciting things for me: the Sega CD is the more unusual original release. I believe it is also known to be more finicky than the second version, but from a collector standpoint I am happy and it seems to work well (no more jiggling of connections than a well-loved cartridge would need). I also got a pristine copy of Sonic CD in the bundle - this game is awesome: enhanced audio (not that I ever found mega drive cartridge releases of sonic to have anything less than excellent sound for what they were), some neat visual enhancements, different time "dimensions" for each level, and the ability to save (taking advantage of save-game space on the Sega CD)!
A neat arcade side-scrolling shooter Sol-Feace also came with it, which seems to have promise.
Right now I'm enjoying the novelty of listening to a Led Zeppelin cd through a 20 year old cartridge-based video game system from my childhood:
I have also discovered that the Genesis and Sega CD, when hooked up to my HDTV via the RF cable, function as a very effective amplified antenna - when powered off, nothing, but when powered on, my over-the-air reception of digital tv is easily as good as it is when I have my amplified antenna hooked up (to clarify, though, your TV must still have a built in digital tuner for this to work ;-) )