It's a solid 9 from me.
I very nearly didn't buy this, as I thought it was just the nostalgia talking. But I picked it up on a whim, and after a short while I felt like it was a real guilty pleasure.
Then I played it some more and realised this is the most fun I've had with a beat 'em up in years, hands down.
The amount of kontent (sorry!) here is excellent as well. Story mode was actually interesting in a beat 'em up for once - although as stories go, it's still pretty damn silly.
Also, Raiden is such a complete douchbag in this game, seriously
The fighting is great, with a well balanced cast and some fantastic stages. Visuals are great in-fight (if a little rough in cutscenes) and animation is smooth.
An extra shout has to go out to the sound effects too, very good.
The Challenge Tower has some genuine comedy to it in places, which is very refreshing - although I guess Mortal Kombat has never taken itself too seriously.
The only reason it doesn't get a 10 from me, is that I'm not a huge beat 'em up fan really, so this kind of game doesn't hold as much longevity for me as other games would. As great as this is, and as much content as it has, I can tell already that it'll be the same. However, I can very much see it as one I keep coming back to for a quality ruck every now and then.