This is really late but I'd decided to go with someone to see it and took a while to both not be busy. The set up of this film is that a group of actors (who we are introduced to at the beginning through trailers of their supposed films) are shooting a war film based on the tell-all novel written by Nick Nolte's character.
Ben Stiller plays the action hero in need of a hit, Jack Black the comedy actor doing something serious and Robert Downey Jr. the ultra method actor who has had his skin darkened to play a black character (this is not offensive in the least, the whole point is how stupid he is for doing it). After egos on set bring the wrath of the studio head (played hilariously by a bald, out of shape, hairy Tom Cruise) Nolte persuades the director (Steve Coogan) to set up lots of cameras and explosives in the jungle and send his actors into "the shit", making it a "real" experience for them and capturing their genuine emotions. Coogan however steps on a landmine 15 minutes into the film and a real attack from heroin-producing natives is mistaken by Stiller for the method of making the film.
The acting is the main thing to talk about here, Stiller does well, he's not over the top and he suits the role rather nicely. Downey Jr. is excellent as the ridiculous Australian actor and continually steals the show. Jack Black has the least to do of the three and the worst written character/storyline but he has a couple of cracking laugh-out-loud moments so he somewhat saves it. Nolte is very good in an extremely underwritten role and Cruise hasn't had this much fun or been as good since Magnolia, inspired stuff. They even manage to make Matthew McConaughey look decent in the role of Stiller's agent (on a mission to ensure the clause in the film's contract guaranteeing Tivo on set for Stiller is honoured), that's just remarkable.
The film is at its best when it's taking edgy shots at Hollywood, criticising the oscar begging, the egos, what have you. Where it's less successful is in the general shots (studios putting profit over people, basic acting stuff, etc.) and that the action side of it doesn't involve as much as it could do because it's a comedy. It is consistently entertaining, but there are stretches without laughs and sometimes they overkill certain things (Cruise dancing gets very old, very fast, for example). I would recommend this because it is funny, pacey, it does exactly what it says on the tin, not a cinematic masterpiece but has a lot going for it.