I thought it was funny, but not nearly the movie that Borat was.
I think the difference is in the characters. Borat, for all his boorish and harassing behavior, was a sympathetic character. He's not a bad person by nature, he's just from a different culture and hasn't adjusted. You rooted for him. Bruno, on the other hand, is a douche. He's vacuous and can't rally the audience around him. It makes his ambush stunts feel mean-spirited, while Borat could pull them off.
Another reason Bruno suffered was that the stunts were more staged than in Borat. You can't really blame the filmmakers, because his fame makes it harder than it was before, but it still makes for a lesser movie.
That said, I thought a few parts were really funny, like the gay sex montage, the focus group, and the scene with the cameos (which I won't ruin.)