UMK3 adds the characters that needed to be in the first MK3, but it's too late now; they already lost too many fans.
MK2 + MK3 = UMK3.
So UMK3 = okay.
Medium length review:
I already reviewed MK3, so I'm not going to say what I've already about the game. No big changes to the game play, so all I can really comment on are the new characters and the new stages. Everything else isn't significant enough to make the game worthy.
Stages:
A hell of a lot more interesting than MK3. Some of them brought that good MK feeling back to fans. Stages like The Subway, The Bank, and The Bridge just didn't cut it. Scorpion's Lair, The Kave, and The Lost (blue mk3 portal with MK2 bridge) brought that good feeling to you.
Those aren't the only stages that are new, so I'll leave it up to you to play the game and found out what they are and realize what I'm talking about.
New (returning) Characters:
Reptile, Scorpion, Kitana, Mileena, Jade... why the HELL were they left out of MK3 in the first place? The morons who made MK3 realized their mistake and put them back in, but it's too late now. So many turned their heads to better fighting games... that were THREE DIMENSIONAL.
Aside from that, they did a descent job with character moves, but for some they god rid of moves and didn't add new ones to make the characters more interesting. Their combos however were great; a lot better than the other MK3 characters in my opinion.
I was glad to see the classic Sub-Zero return, because the younger brother looks like some damn American in a torn up clown suit.
This game is great for only new characters, but it still has all the crap MK3 had (and when I say crap, you should read my review on MK3 to find out what I am talking about)