It's been a good ten years since the last Marvel vs. Capcom game. And I'm happy to say it's been worth the wait.
First thing to get out of the way, right off the bat, is that the control scheme is different from the last installment. Those that have played Tatsunoko vs. Capcom for the Wii know what they're getting into, but if the last Vs. game you've played is Marvel vs. Capcom 2, it'll take you a while to adjust to the change. Hell, it took me 20 minutes just to figure out how to tag out.
Once you get used to the controls, it's all awesome from there! You're still going to run into the occasional Tournament-level Expert, but for the most part, the playing field has evened significantly. It's pretty easy to make a comeback, especially once you learn how to use the X Factor ability effectively. The X Factor ability (accessed by hitting all four face buttons) gives you a small period in which your health regenerates and your attack power increases proportionate to how down and out you are. Once you get used to using it, it's a life saver.
There's fan service to be found everywhere! Stages are plucked from Marvel locales (S.H.I.E.L.D. Hellicarrier, The Hand's hideout) and Capcom hotspots (The Demon Village from Ghosts 'n Goblins, Resident Evil lab). But more than anything, the dialogue is an absolute hoot! Characters will often spout out specific dialogue depending on who they're facing at the start of the match, at the end of the match, and in the word balloon at the end. These interactions are often hilarious, whether it's Captain America telling Zero "Your name is Zero? Hope you don't fight like one", Iron Man posing victoriously and telling a fallen female fighter "Told ya. Candlelight dinner would have been nice", or Deadpool saying...well...ANYTHING.
If you're a fan of the series, you want this game. If you're a Marvel comics fan, you want this game. If you're a fan of fighting games...well...it'll depend on your preference. This is a much more button-mashing oriented game than Super Street Fighter IV, so it's your call.
Thumbs WAY up for Marvel vs. Capcom 3! Now let's bring on the DLC characters!
I still can not get used to using O for fireballs and such. O has always been my fierce kick button.
Unless you have a level 5 bar, don't even try to use Phoenix. Her Dark Phoenix w/ X-factor is deadly, but the X-factor goes up in power of you only have 1 character left. She has such a glass jaw, it's not even funny.
Dormammu plays a bit like Blackheart (but a tad deeper) and is my new fav. According to my play data, I've used about half of characters at least once and I'm not a fan of Chris or Spencer. Actually, I barely use any of the Capcom fighters for some reason.