The Ali G character works brilliantly within the confines of a comedy show, but as a movie, it doesn't work in the same way.
Don't get me wrong - this is a very funny movie, full of biting, witty dialogue, that caricatures the modern British chav wonderfully well, whilst providing the viewer with a hilarious, if unrealistic story.
One problem with this film is that the script and content is either fantastically brilliant, or it's embarrassing to watch. When I say embarrassing, I don't mean funny embarrassing a la Office or Extras, but rather, you'll wish they hadn't included it in the final cut. One example of this is the inclusion of a music video after the film has ended, to the tune of, "This is how we do it." Whenever I watch the film, I stop the DVD when it says the end, and leave it at that.
Overall, Ali G Indahouse is a good film, worth watching a couple of times. The script is enjoyable to an extent, and there are no issues as far as acting goes. However, refinement is the key word here.
Ali G is a better television programme. Borat is a better film.