Just borrow my brother's game and played some of it. I haven't played any of the story mode, so this isn't a review of any kind. I'm just pointing out my opinions about the core fighting so far.
I have to say...it's pretty good, but not much more than that.
Here are some pros and cons for me:
+) Good fighting mechanics that are fun much in the same way as Mortal Kombat.
+) Some cool combos.
+) Good use of the health bar instead of simply having two "rounds".
+) Great stage transitions, even though some are like "That should kill him/her".
-) Some questionable choices in character selection. I mean, is Bane really needed in this game? There are too many slow, big brutes in the villain side. Could have used this slot to someone else.
-) Some pretty bad redesigns.
-) The fighting is too unspectacular given that it's a superhero fighting game. I mean, it is fun and all that but I never feel like a super powered being in this game. Capcom captures this SO much better in their MvC games. This game is simply too slow and stale. This is one of my main problems with this game. It feels too unimaginative given the great characters they have in use.
-) Some super attacks (full bar) are cool but most of them are simply weird and boring to me. Again, Capcom does this kind of thing much better.
-) The game is pretty imbalanced it seems, with characters such as Deathstroke and Sinestro. I've heard they are overused online too.
-) Boring graphics.
Now, as I said, I haven't played story mode or any of the other stuff. From experience with Mortal Kombat 2011 and from what I've heard about this game, the story and bonus stuff doesn't seem to disappoint so I bet those would be on the plus side.
Overall, from playing practice and fighting mode only, this feels like a good game mechanics wise looking at it simply as a fighting game. But at the same time, one would expect a lot more given the characters and the DC world they play in. I can't get over how stale and unimaginative these characters feel to play as, some boring character choices, the dull graphics etc. It's still pretty fun to play in its core though.
Some people said as a fact that this game and Mortal Kombat 2011 are the fighting game highlights of past generation. Well, I agree that Mortal Kombat is one of them. But I'd rank SSFIV and Tekken Tag Tournament 2 above this game (as well as most reviewers as well do, from what I've read). I'd rank MvC3 higher if it wasn't for how bare bones that game is, and UMvC3 seems to have fixed that.
I'd say I agree with the general critical consensus that this seems to be a 7 or 7.5 out of 10. Good but not great. I'd much rather see a Marvel vs DC made by Capcom.