This is definitely one of the funniest movies I've ever seen, and, like other Mike Myers movies, becomes even better with repeated viewings. If you liked Austin Powers or Wayne's World, you will like this, although the humor is sometimes on a somewhat different level -- not always more elevated, but more subtle. But many parts will be funny even if you don't pick up on the subtleties.
This movie gets a lot of flak from critics for its ending and its structure, but nitpicking about these aspects (which don't seem to me to be flaws at all) is missing the point of the whole thing. Go into it expecting only to laugh and you will be extremely impressed.