"Hot Tub Time Machine" sounds exactly like what it is: a stupid and yet brilliant premise. This is one of the best stupid movies (defined as a movie featuring projectile vomiting on rodents or some similarly horrendous sight gag) I've seen in a good long while and, he bonus, a bunch of references to the eighties. Which, if The Wedding Singer has proven anything, it's that when you set your movie in the 80's you are bound to hit on at least something funny.
If you are looking for a brainy review of the movie, including meditations on the existential implications for those suffering through the disappointments of life, etc. etc. then go here.
Otherwise, I hereby grant Hot Tub Time Machine the Nat Topping Seal of Approval!