This time around, there is a lot of focus on 3D effects and action. I doubt it will have much appeal in 2D, so your best option is to go for 3D.
The plot is totally stupid. If Scrat the squirrel in a flying saucer, an asteroid on collision course with Earth, and the most ridiculous mission to prevent certain doom, was the best the writers could come up with, then it's really perfectly fine, if this is indeed the last movie in the franchise.
While this is easily the weakest of the five Ice Age movies, it's not all bad. It's impossible not to like the group of friends, who have all been thru so much together. All the usual suspects are here, and Buck the one eyed weasel, is back from the underground to help our friends save the world.
If you loved the other movies, just go see it, but don't expect to be blown away from neither the movie nor the asteroid.