Without going into spoilers, I'll try to focus on the things I really liked about this movie: 1) The cast is freaking hilarious, like a lot of comedies since There's Something About Mary, there is a lot of gross out humour, but also like that film, the story invests you in the characters, so you can empathize with them throughout. The real stand out for my was Rob Corddry - as the most obnoxious of the four, his character was either going to be loved or hated - no middle ground. I loved him. 2) The story is internally consistent, this is really a key point for me in Time Travel movies - in fact that's one of the reasons I love the Back to the Future trilogy, they made the internal logic consistent. As a genre fan I'm willing to buy any outrageous concept, but you have to play by the rules if you want the story to work.
In the end, would I recommend this to friends - absolutely (the film is definitely worth it's 18A rating - so no kids), would I watch it again - yes, but on DVD, and finally would I buy my own copy on DVD? Absolutely.