I watched it yesterday and LOVED it!
Pacific Rim is the movie I've been waiting for while I was watching all those spectacular, but soulless Michael Bay, Roland Emmerich, and Zack Snyder movies. In it, Del Toro delivers one of the greatest examples of sci-fi world building I've seen in cinema. The attention to detail is incredible and he nails the sense of scale and weight that so many similar movies lack.
That said, it's probably not for everyone. If two hours of giant robots hitting giant monsters in spectacular ways doesn't sound like fun to you, Pacific Rim might not be your cup of tea. It's basically an extremely nerdy movie, which is also why I'm not hugely surprised that it isn't doing too well at the box office. But if you're anything like me, your inner 10 year-old is going to lose his (or her) mind and fall madly in love with this movie.
As for the 3D: Considering the movie was shot in 2D and Del Toro didn't even want it to be 3D, it's remarkable that the 3D is among the best I've seen, on par with the likes of Avatar, Hugo, and Life of Pi.