I love Smash Up. My group plays this fairly regularly, sometimes as the main game for the night, and sometimes just as a palate cleanser after a really complicated strategy game.
We've come up with some variations on the rules, including team play and draft modes. I think we'll be playing this one for a long, long time. I highly recommend it. (I also recommend picking up some sleeves for them. Ultra Pro sleeves for magic cards fit fine in the original box, Dragon Shields are a little snug. If you get the Big Geeky Box expansion, then either fit great.)
My only gripe with the game, is that once you add in the expansions, a few of the deck combinations get a little too strong. For example, Zombies and Geeks feels nearly broken, and our group frowns on someone taking them together.