I won't even blow up about MK's online issues anymore, but its so sad really...
If you're looking for mostly online play out of it then don't buy it. I obsess over mortal kombat and love it to death, but not being able to pull off half your characters moves due to lag and sporadic timing changes makes is 0% fun, unless of course you dont care if you win, or just like to punch and kick the opponent.
I went out and bought Street Fighter IV: Arcade Edition literally last night because I wanted to beat the shit out of ppl online (without lag) and it was 100% perfect. There's not much but online in street fighter, but i think the online modes are better and more fun in Street Fighter.
And if anyone else is playin SFIV, feel free to add me on xbox, i dont know ANYONE that plays it.