Crypt of the Ncrodancer was a bit of a disappointment for the reason stated by Darkmere. While the music is generally very nice (you can look it up on Youtube) it doesn't actually bring anything to the game. While most games incorporate the music into gameplay (even in minor ways such as stepping to eigth notes in DDR) in CotND you're stuck with just simply doing every action on the beat of the music (which is all 4/4, by the way, as far as I can tell). The difficulty comes from the more fast-paced songs, leaving you less time to react to enemies. My favorite boss in the game is King Conga simply because there is a slight integration with the (awesome) music, as every 8th beat is a rest, but that's about it. It also wasn't really designed to be used with a controller it seems, but you can use a dance mat if you wish which I imagine might make the value of the gameplay go up a bit if you can afford to have such a peripheral lying around when you're not using it.
TLDR: Good music, poor integration of music in the gameplay of a music-based game. You'll like it the first few times you play, but that's about it. I suppose I was hoping for more rhythm game and less roguelike.