Despite my initial lukewarm reaction to the concept, I picked up MK vs DC Universe yesterday after checking out some previews and I have to say, I like it. It plays very similar to the MK games on the PS2 and earlier in terms of the high punch/low punch, high kick/low kick and special move combinations which is bad news for anyone who dislikes the simplicity of the series but it's something I've always enjoyed about Mortal Kombat so I'm glad they didn't tamper with it too much. They seem to have kept the useful things (like combo breakers), discarded the things you can do without (weapons, though some characters use weapons as part of their special moves) and added some cool new things (like the free fall kombat).
The DC influence isn't the downer I thought it would be. The fatalities, while not as bloody and violent as some of the ones we're accustomed to seeing, are still within the MK ballpark and probably about as far as DC was willing to go with them. That part of the game is a lot better than I thought it would be and it preserves the dark atmosphere of the game, which I realise comics do have but they have toned the gore down a bit. So far the story mode is pretty entertaining and I like that they kept the tournament in the game, with the option of playing against MK characters, DC characters or a mix of both.
If you're a fan of MK and aren't looking for a great deal of innovation - controls are basically the same kind of layout and the same kind of strategies come into play - it's a good game, with the DC characters far less intrusive than I thought they would be. Having said that, I would prefer if the next game stuck to the MK universe and found a way to continue or maybe even reboot/slightly retcon the storyline they've established. The crossover doesn't seem too bad though, happily I was wrong about that.