The Borat movie was great, but the Ali G movie was terrible. I think he learned his lesson from Ali G not to stray too far from the TV format that makes his show funny, even when doing film, which is why Borat was awesome. If Bruno's not as good it will be for one of two reasons-- 1. This character, in my opinion, while funny, was always the weakest of the three or 2. We've seen so much of Cohen's shtick at this point, the new stuff may lose something.
What I did not like about Borat, was that you could not tell the difference between the faked scenes (people playing dumb) vs the real ones (people being played for dumb). In fact from what I remember most of the stuff was apparently faked. Thats why I prefer his TV stuff.