A lot depends on what your personal preferences/needs are. I used to go all MD for portable audio, and I still think MD is a great format. However, I became frustrated that I wasn't able to carry around more than 12 albums with me without a hassle, and recording and labelling MDs was a bothersome process. So I decided to go for an MP3 CD player and got a RioVolt, and I must say that it's suited my needs VERY well. Carrying around 24 CDs gives me access to 240 hours or so of good quality VBR mp3s, so my unpredictable musical cravings are almost always met.
Anyway, just thought you'd like to hear the perspective of an MD fan gone MP3. If you've already got an MP3 collection and a CD burner, you might want to look into MP3 CD players as an alternative.