I finished this the other week. Hot damn, that was fun to play through again. Even the secret boss from the original is in there, and exactly the same. Once you've finished the game, you can access, and challenge more powerful bosses that you've fought previously, and they are tough. It all culminates with another fight against Culex, but an updated, polygon, 3d version that is tougher than the previous versions. It's a damn good fight.
If you haven't checked it out, please do. This is a perfect remake of an SNES classic.