Nowhere near as good as Gladiator! Brad Pitt is no Russell Crowe! I can't help but compare this to 'Gladiator' and find 'Troy' sadly wanting. 'Gladiator' was one of my favorite films of recent years and I thought Russell Crowe was magnificent as Maximus. (Sure the film had some issues, but the good far outweighs the bad for me). You absolutely believed him as a soldier and general and a man of the ancient world.
The same cannot be said of 'Troy.' Brad Pitt as Achilles is the big Achilles heel of the film. (Sorry, couldn't resist). He doesn't have the necessary heft as an actor and seems to expect his newly muscled physique to be enough. It isn't. He's fine when he's in combat scenes, but whenever he opens his mouth, his "now you hear it, now you don't" British accent is embarrassing. Especially alongside real British actors, like the always excellent Sean Bean. (Thank goodness Bean gets to provide the opening and closing narration as Odysseus. Can't get enough of his marvelous voice!)
Apart from Brad's woeful acting is the presentation of the character of Achilles as a very unheroic hero. He's basically a glory-seeking jerk, all about individual acclaim and definitely not a team player, kind of the Kobe Bryant of the Greeks. He doesn't care about country or king (true, his king is corrupt), but compared to the noble and far superior character of Hector, we instinctively root for Hector. Eric Bana as Hector is the real hero of this film and whenever he's on screen, the movie actually works. Eric, all is forgiven for "The Hulk!"
To paraphrase a Gladiator quote (as this film does freely in its familiar dialogue), Achilles has the strength, but Hector has the honor. At one point, one of Achilles' men tells him, "To serve you has been the greatest honor of my life," but we never see why. Sure he's a great warrior, but he's disrespectful of all authority and frankly a self-centered brat. All he cares about his himself and his cousin. Hector is the true leader here, making the hard choices and taking the brunt of the fallout from Paris' rash decision to take Helen to Troy.
The rest of the cast is mostly good. Orlando Bloom plays Paris as an experienced ladies' man but inexperienced and frankly bad warrior, which worked for me. Some of his dialogue is naive and laughable, but I think people were laughing at the character, not the actor.
The only other actor I had a problem with was the girl who plays Briseis. She was extremely annoying and her whole love storyline with Brad Pitt was completely uninvolving and only slowed down the film.
And I'm not an expert on the Iliad, but I do know that Agamemnon meets with a different end than in this film. Read the play 'Elektra!'
Overall, I give the film a 7/10. Worth seeing, if you can tolerate Brad Pitt for nearly three hours. By the end of the film, I was actually cheering for him to be taken down, I hated him and his character that much. And did they ever explain why Achilles is invincible? If they did, I missed it. I think they should have explained it for the majority of people who are unfamiliar with The Iliad.