Saw this in IMAX. For this film, IMAX was not necessary. In fact, you won't be missing much if you see it at home, on the small screen.
If you're going for the special effects, you've probably seen these before. You'll recognize some from The Mummy especially.
If you're going for the cast, you won't be disappointed. Keanu Reeves is a reasonably convincing Klatu, while Kathy Bates is at her best, stealing the show. Jennifer Connelly does well, delivering a more developed Helen than seen in the original 1951 film. Oh, and it's fun to see John Cleese, if only briefly.
If, on the other hand, you're going for the plot, the original tells the story much better. There are some changes that I strongly disliked. The actions taken on the part of Klatu are mostly the same, though the underlying reason is somewhat different, though not altogether unwelcome. The ending chosen was needed, but shouldn't have been left for the end. For fullest appreciation of this film's plot, don't see the original... at all. For the fullest, most thought provoking experience, see both movies.