I thought the original, British Ali G shows were genius. The US shows were just rehashed setups and weren't as funny for that reason, but also because the manner of his characters were made more exaggerated and "obvious", which I didn't like.
The Ali G movies, and i'm not even sure if i've seen them all, sucked.
Borat, yes, very funny, but there were just too many staged scenes, and again, the exaggeration was ramped up even more. Check out "Best of Borat" at YouTube, which are all scenes from the British Ali G Show.
If Bruno is supposed to be even more staged than Borat, then i'll automatically like it less. I still expect to laugh my ass off though.
I've seen Borat twice. Once at the cinema, once at home, alone. I found it quite a bit funnier the second time, probably because I went in accepting of all the staged setups. And, to touch on what others have said, I prefer to dictate the laughs by myself, and not by a cinema full of hysterical teens.