Castlevania Adventure is not that bad, I like it. It actually got good reviews when it came out and was very popular. It lead the Nintendo Power play list for a few months. And remember: this was the only way to play Castlevania in the car.
in the game you will quickly get to a part where there is a long flat field with stones you have to jump across. If you fall, you have to go left and restart. I think everyone that gets to this part gives up. Yes, the controls suck, but so do most platformers on the gameboy.
The game is linear with no saving, so it is a trek.
But there are some great parts also:
Music is phenomenal
Bosses are a fun challenge
Graphics aren't that bad considering the age
Flame whip!
The next Castlevania on Gameboy, Belmont's Revenge improves on everything except music. It even has stage selection and passwords.
The final one, Legends is so bad it is non-canon. Bad music, pointless pit-traps, bosses that just stand there.
Pretty neat how there were three Gameboy, three GBA, and three DS Castlevanias (not counting kid dracula, the double-pack, or NES classics).