I saw this last night and it was amazing. The whole 3D IMAX experience was amazing, and I felt totally immersed. The special effects and overall visual aspect of the movie are excellent. The plot has been done before, but I still felt involved with the characters.
Before I saw Transformers 2, I was hearing a lot of negative reviews, mostly saying there were too many comic relief characters, and not much plot. Personally, I don't need much plot to be entertained. Transformers 2 completely fulfilled my expectations, those being big explosions, giant robot battles, and snarky remarks. I thought the comic relief was fitting because I didn't think it was tied to any particular characters, as they all had funny remarks at one point or another. I felt that stayed with the style of the first Transformers.
I don't think you should expect an amazing movie, but it is a really good summer blockbuster.