The inevitable comparisons between Tropic Thunder and Apocalypse Now are somewhat misguided, I feel, since at the very heart of Ben Stiller's magnum opus is the desire to take the piss out of that very film. One should not compare a parody with its target on an artistic level. It's supposed to be similar, and therefore, by its very nature, denies creative comparison.
A similar mistake was made recently when Total Guitar Magazine copped a brain fart and included the Mike Flowers Pops spoof of Wonderwall on its worst cover list; Vegas-style renditions, a la Flowers or the brilliant, aptly named Richard Cheese, are not meant to be good. It's just a bonus if they are (case in point: Cheese's stellar cover of Duran Duran's Hungry Like the Wolf).
Back to the film, luckily, Tropic Thunder is good. Funny in places, most of the time thanks to Robert Downey Jr. Serious in places, interestingly thanks to Jack Black. Hilarious in its pisstake of pretty much every serious war flick since, and including, Apocalypse Now.