Good stuff:
Awesome threesome
You get not one, not two, but three games in one. At first, only the 2.5D remake of Castlevania: Rondo of Blood is available, but after finding two secret items you'll be able to play both the original RoB, and the PSone classic Symphony of the Night. If you're worried about not getting enough bang for you buck, stop worrying right now.
A foot-massage for your ears
The Castlevania series has always been known for it's great music, and this is no exception. Symphony of the Night has what is possibly the best video game soundtrack ever, ranging from some rock/metal tracks for boss fights to some epic orchestral pieces when scouring the castle's corridors. The RoB remake has some cool remixes of the original's great music. By finding hidden CDs in the remake, you can unlock the complete soundtracks of all three games. You can set unlocked tracks to different stages in the remake.
Take it easy or get it hard-on
The old-school Castlevania are also known for their difficulty. Both versions of Rondo of Blood are HARD. Like nail-biting, hair-tearing hard. Their difficulty easily rivals games like Mega Man. Symphony of the Night, on the other hand, is quite easy. You may die a few times, but the difficulty is nowhere near frustrating. This makes for a great balance - if you'd like a real challenge you can play Rondo of Blood, and when you're not in the mood you can play SotN.
The pinnacle of a genre
Buying this game just for Symphony of the Night is definitely worth it. The game is by many, including me, considered the pinnacle of the series and the best 2D side-scroller ever. The various areas in the gothic castle look great, and all animations are fluid. The boss fights are really cool, and RPG aspects like leveling and the abundance of different weapons, armor and shields blend into the game really well. The game is pretty big too - completing the entire game(the regular castle, and the the inverted version) takes about 20 hours. After that, you can play through the game again using alternate characters.
Bad stuff:
Bondaged at first
You have to unlock the two classic games in the remake. Finding the secret items that unlock them can be quite tricky - if you're like me and can't wait to play them, you'll check an online guide.
Hard turn-off
The difficulty of Rondo of Blood might put some players off. The character isn't nearly as maneuverable as the ones in Symphony of the Night and more recent Castlevania games, which can be frustrating.