As far as gameboy games go - Belmonts Revenge runs and plays very well, frankly if it was colourized and has its resolution changed I wouldn't be surprised if it was from the NES.
As well as that, the core gameplay is ok - but hard to say anything about as it's the same as all the others, except the ability to play the first four levels in any order.
With that said, I thought that the final 2 bosses were rather poorly designed. The 2nd to last had a couple attacks that I truly do not think can be predicted or avoided and Dracula just doesn't telegraph anything - trial and error is required and I'd recommend playing the game on an emulator or the Castlevania Anniversary Collection for the sake of save states thanks to that boss alone. (If you're a purist pretend you didn't hear that).
Overall, Castlevania II is a massive improvement over the slop fest that is Castlevania: The Adventure and really proves the worth of the gameboy as a platform, in spite of some questionable design choices and slowdown.