I must've seen this movie a hundred times, and it's still on my top 10 list. It truly is laugh-a-minute stuff for anyone with even a vague sense of humor, while the gags run the gamut from ridiculous to sublime, the plot twists ands turns, and the characters are actually well-developed and distinctive, more so than you'd expect for what is essentially, a romantic comedy. In my opinion too much attention has been paid to various scenes in the movie (I'm talking about clearing the pipes here...) and that shouldn't overshadow the many other hilarious bits. I think it definitely is a movie you need to watch repeatedly to pick up on all the running jokes and more subtle humor.
It also helps that Cameron Diaz is pretty much every man's dream - beautiful, low maintenance and willing to date nerds. Ker-ching!
Stiller is fabulous as usual, gamely getting humiliated movie after movie, but I think this movie was his zenith, the peak of his powers and so-on. he brings a smile to our faces, and at least once, tears to our eyes.
The supporting cast is also used effectively, with Lee Evans and Matt Damon in particular standing out.
The Farrelly brothers are the kings of skating the thin line between good and bad taste, funny and irritating. Here they come down solidly on the side of solid gold entertainment, although it appears many people here beg to differ as to which side of the line that is!
I think I'll go hit that 101st viewing now...