Is it really that much better? Damn. I played Castlvania 64 a lot back in the day and I didn't hate it as much as most I think, but I never tried the new version because I just thought it would be mostly the same with just another character. I guess I should look it up on youtube. I'm guessing it's expensive now on ebay, though...
Also, I like Twin Snakes more than the OG version, but it feels weird calling it a "director's cut" since it had a different director. Though I guess Kojima did sign off on it, from what I understand. Still my favorite MGS game...
I'll say this... the game is still very flawed, but that's mostly due to how it's dated and aged over time (it's been over 16 years, after all), though it still had some rough patches there even at release. It's not exactly a homerun, but I found it to be far better than reputation would imply (and most of that reputation is based on the original Castlevania 64).
BUT, in my opinion, compared to the original, it's a gargantuan improvement in every area. Let me summarize:
+ 2 new playable characters, both with unique mechanics (a transforming werewolf and a shotgun-totting knight)
+ New stories, new cutscenes/characters
+ 4 new stages, plus many of the original stages from C64 have been improved and redesigned
+ 8+ new bosses
+ 2 new costumes for every character
+ new minor enemies
+ new weapon effects and particle effects added
+ new music (sinking old sanctuary remix is great)
+ new sub-weapon system
+ slightly sharper graphics (even without the expansion pack)
+ tighter controls and better jumping/platforming mechanics
+ more responsive, more customizable, more reliable, less buggy camera with better angles
+ Bugs/glitches fixed from the original
- Malus' Violin song from the original is missing
- Giant skeleton jump scare removed
- sparkling snow crystals removed from Tower of Sorcery
- No voice acting except for Cornell's intro narration
But you can tell this was meant to be the true "definitive" version of Castlevania 64. The original Castlevania 64 campaigns are unlocked in the game in their entirety with additional new stages, bosses, and mechanics as well.
Also, the game isn't that expensive. I bought my copy awhile back on ebay for around $30, complete in box with instructions.