I had a CP4 for years. I loved the action, the CFX samples, and all of the electric piano sounds. The latter actually included some physical modeling parameters like hammer attack that were pretty fun. You do a lot of menu diving, and then scroll wheel turning. If you are only dealing with a single parameter live, not that bad. If you expect to change multiple parameters on the fly, forget it. I have a YC61 now, and it’s a world easier to tweak effects.
Pluses:
+ pianos sounded great
+ great action, especially for Rhodes sounds
+ insanely light
Negatives:
- parameters were all menu diving on a tiny screen
- body is mostly plastic, hence the weight savings
- no escapement
- limited resonance modeling