Whaddya mean there's no tag-team?
The most innovative feature of X-Men vs. Street Fighter in the arcade was the ability to fight tag-team with two people. Besides being able to switch between them during combat, you could also execute super moves utilizing both characters. In the PlayStation version, you can only do one of those two, the dual super moves, whilst the tag-team mode is completely gone.
As an arcade conversion, X-Men vs. Street Fighter is fairly poor. It doesn't mimic the arcade version correctly - plenty of dropped frames of animation, a complete lack of the tag-team mode, and a slow frame rate are just a few of the various problems.
Just as Capcom seems unable to ever come up with a new name or setting, the gameplay is the same as all other Capcom fighters. It plays just like one of their classic 1 on 1 games except that it has the new combo system. It's not that the game isn't fun, it's just that it doesn't play much like the arcade.
The graphics also really show the limits of the PlayStation in terms of the amount of RAM and ability to draw complex high resolution sprites. X-Men vs. SF, a game that was silky smooth in the arcade, can be chunky and really lack in the frame rate department. Also, because the PlayStation only has 2 megs of RAM, massive numbers of character frames had to be cut.
It's not that a CD couldn't store all the frames, it's just that all the frames necessary for animation couldn't be stored at any one time in the small system RAM. The more popular characters have retained more frames (Ryu, for example, has the most) while the less popular (and much larger) have next to nothing in terms of animation (Juggernaut, for example). Graphically X-Men vs. SF is an extremely poor conversion.
In terms of gameplay, X-Men vs. SF plays a lot like Street Fighter Alpha. It has all the same moves as Alpha, just adding the super move ability and the larger vertical screen size. Combos are, for the most part, easy to learn, but hard to master - which adds a lot of depth to the game. Gameplay is good, but we've seen it all before, warranting purchase to only the most hard-core of Street Fighter fans.
For those who aren't familiar with the game, the aforementioned super move system is the only real different between X-Men vs. SF and previous Street Fighter games. After building up your "super meter" you can execute moves that do massive amounts of damage. You can even use your tag-team partner to do two super moves at the same tame, causing even more damage. The strategy involved in using these moves (they can be blocked, which renders them wasteful) is another part of the game that will take you a while to use correctly. Based on the number of different super moves (each character has two), you'll have to spend some serious time learning how to use each correctly.
X-Men vs. Street Fighter is a fun game, but it is a poor conversion of its arcade counterpart. The graphics aren't going to impress anyone either. If you're looking for a game that is a better arcade translation, check out Tekken 3, or wait for Street Fighter vs. the Care Bears, due out later this year.