I just got back from seeing it in 3d Imax. I loved it. I went in looking for giant monsters versus giant robots, and that's what I got *with* a good cast of characters and some interesting backstories. I though the 3d was ok. Although Imax always makes ok 3d look great.
My favorite characters were actually the two scientists whom I think did a great job of advancing the plot. Although all the main characters were great. My only real complaints were that we really could have used a good daytime combat scene, and somewhat connected, the final battle scene was a little bit confusing because of the darkness and the kaiju involved were a little too same-y(which wasn't the case in the battles beforehand). At least on this viewing. I might change my mind on that last point on a re-watch.
I think Del Toro nailed the subject matter in a way that will make the true fans and the summer popcorn movie crowd happy, which was what I really wanted to see from this movie. Hopefully, this will draw some people out of the latter crowd and into the former to help soar our ranks.

Del Toro is definitely a fan, and I could tell immediately once I saw the cast on screen in action. Imagine if Michael Bay had cast this? Ouch. It makes my brain hurt.
I haven't commented much in this thread about this movie as its progressed to its release except to say that one of my hopes is that the movie would help warm me to the jaeger and kaiju designs. I think Del Toro accomplished that. Put in an order for the NECA figures.
Spoilers:
My only real disappointment is that we didn't get to see enough of the Crimson Typhoon and Cherno Alpha crews and their jaegers in action.
And I thought it a little bit weak that a nuclear blast didn't kill the Cat 5, but it could kill the kaiju aliens on the other side of the portal? And like someone else said upthread, the Cat 5 really didn't feel like a Cat 5.