I saw it with two friends a couple hours ago. I'd say Tropic Thunder is mostly Above Average.
I didn't think that the fake trailers were all that funny; the Tarantino/Rodrogiuez Grindhouse collaboration used fake trailers in a more effective manner.
Grindhouse's trailers were done to keep in tone of the "Grindhouse" feel of the movie. Tropic Thunder's were done to establish the characters and the absurdity of Hollywood.
Big differences between the two.
I have to say that I don't think the fake previews would have been as effective had I seen the movie normally. There were no regular previews in the screening but I think most in the theater were expecting them. So when that ad for the ass drink popped up everyone burst out laughing and was like, "wtf? Is this real?" It was honestly pretty hard to tell! It wasn't until the next fake preview that we realized it was part of the movie.
The theater's "Feature Presentation" bumper didn't tip you off?
And who added those Tags?! As if the thread title wasn't obvious enough?
"Tropic Thunder Grading and Discussio, hmmm wonder what that thread is about? Well, let's check the tags..."