Gamespot hit the nail on the head, possibly the best 2-D side-scroller ever.
This is the absolute high point of the Castlevania series, and that's saying an awful awful lot. It had perhaps the most interesting main character, made a great transition from the old school games to the new age, and had a free roaming map that was, in a word, "enormous". Not only that, if you managed to play it correctly, you got to experience the whole map yet again, in upside down land, and the fun just kept going and going. It's probably not the hardest in the series, what with the addition of all the recent Nintendo DS titles. Dawn of Sorrow or Order of Ecclesia would probably vie better for that title. And while the old games were often punishingly harder, the main reason for that was the poor controls of the day, which couldn't much be helped.
However, this game somehow managed to both capture the spirit of the older games in ways that its "rival" Castlevania 64, and its "successor" Castlevania:Lament failed to, and yet seemingly invented a brand new style of its own.
Suffice to say, even if you have played 2D side-scrollers before (Some of you younger guys may not have in this day and age), you have never really played a game like this. Even the new DS Castlevania games don't come close to the scope of how big the explorable parts of this game were, nor the secret areas, etc. There are SOOOO many items, weapons, armor, etc. in this game that the NDS version don't even come close to, and the PS2, and N64 versions don't even make an attempt at. The map alone will take you days to explore if you don't know which areas to hit up first, and where to head to next.
The boss fights are also very inspired, some mirroring bosses from earlier Castlevania games, some entirely new. Some easy, some unbelievably hard. (I hate you Galamoth)
The game also features an amazing Soundtrack of original music, even by today's standards.
It's awesome that this game has been brought to modern Venues courtesy of the Xbox 360 store, and PSP's Castlevania: Dracula X Chronicles, so if you own either of these system, and have ever enjoyed a Castlevania game, then get out of your chair and track this thing down ASAP, you'll be glad you did.