I don't think I would give it five donuts out of five, I am a big fan of stupidity in films Jackass and J & silent bob the move style and though Borat was funny it wasn't all that funny.
There was a lot of good material in the film but after around 3/5 to 2/3 the way through it just got to stupid and lost its comedy factor.
Having said that I wouldn't want to discourage anyone from watching it as Borat might be able to entertain you for whole film.
I would give it
just my