The film is well-cast, endearingly so. I personally, like Ebert, liked Zach Galifianakis the most. I also really liked Ken Jeong as Mr. Chow. But there are a lot of people in this film who spout a lot of good jokes. A lot to choose from. It's a very quotable film, as well, which, like with "The Big Lebowski," practically guarantees "cult status" (my friend said that this movie has a good chance of being really remembered, because it was "well-written"). Don't be surprised if a lot of people you know are saying the lines from this film. There are fan pages on Facebook devoted to particular lines of the film. That's a good reason to see this film.
You probably know the plot already. If you don't, I'd actually be very surprised. I actually won't tell you. You have to rent at your own risk. Look at Ebert's review if you want a synopsis. But if you don't know the plot, that's a blessing. My friend calls the plot "surreal," and that's very true. After it gets going, it really gets like that. The trailer gives up jokes, etc., and all other sources do, so just get watching. If you go in without any idea, you may really like it a lot. It's a courtesy to the filmmakers, I guess.
For some reason, I was a little weary of the beginning of the film. Not the prologue, but after that, before the main section of the film. Something about it bugged me. It may have been that I was consistently thinking, as a female friend had said, that this film was "so male." Once it goes into Vegas, etc., it works a little better and gets pretty funny. It "cops (no pun intended) a gag" from "Pineapple Express," but it improves. As the film runs its course, it gets a little giddy, and is also perhaps a little too straightforward, but I guess that's how it should have ended/solved itself. I realize this is remarkably vague, but I'm describing how I felt.
Writing this review a day later, I realize I have more an affinity for the film than before. But I'll weigh my previous feelings in my grade. The acting and writing and humor are good. Altogether, it's not a great film, but it's a "pretty funny" one with some "comedic weight" (as they would call it). B-