Evolution gathers 22 Capcom warriors together to do battle. They hail from five games, and one is actually new. The mildly unusual part is that they all retain their native gameplay styles. So, the four Street Fighter II guys hit hard but lack complexity. The Street Fighter Alpha crowd can use the alpha counter and custom combo. The Street Fighter III contingent parries and has those wacky two-button throws. The four (well, five) Darkstalkers are chain-comboing monsters, but are tempered with weak defense. Last, the characters from the little-seen arcade game Red Earth are just weird, through and through; for example, they can spend meter to permanently level-up in mid-battle.
And battle they do. CFE has you select a two-character team, but you can only swap characters between rounds. Your opponent (if you can coerce a friend into playing) can't tell which of your fighters is coming up next, so there's some small element of luck in choosing counter-characters and such. The fighting is typical Capcom, and the engine feels very similar to Capcom vs. SNK 2. What's not to like?
A better question might be, "Why should we care?" Outside of the Red Earth folks, there's very little novelty to be found in Evolution's anemic cast of 22 warriors. Ryu plays like Ryu. Guile plays like Guile. Demitri plays like Demitri. And so on. Any serious Capcom fan will know these characters inside and out, and since they bring their native game systems with them, there's not too much new stuff to learn. After a few matches as your character of choice you'll be back in the groove and wondering why you're not just playing your old Capcom fighter of choice instead.