Here's the short version of the review: don't bother.
The long version after the break...
Here's the thing about trying to do a Zack Snyder-like film: Zack's a pretty smart guy, and his films are too. Even when there's lots of overly-oiled guys throwing spears around, there's an undercurrent of intelligent analysis of history. 300 was about the way we convince people to go to war. It was a fairy tale about propaganda. Immortals has no such redeeming value, but it seems no one told Tarsem Singh that he was aiming for anything higher than a bunch of virtual sets with slow-motion head-bashing.
This parody of 300 is set after the Titanomachy, and that's pretty much where any pretense of compatibility with actual Greek myth ends. The gods are, of course, arrogant whiners, but the wrinkle is that they appear, in this film, to have no more sway over the universe than your average superhero. They can move very fast and observe events from high atop Mount Olympus, but can't actually make any changes to the course of events without showing up in person to smack humans around. In order to pose a surmountable threat to these toned-down gods, the titans are also reduced to merely fast and strong human-like creatures that seem to be feral and interested only in destruction.
A word about 3D techniques. 3D is all the rage, right now, and from what I can tell, there are some people who are working very hard to refine the technology and figure out how to incorporate it into the art of movie-making. We'll see how that turns out, but so far, I've yet to see a live-action film that I thought it added anything to. People like to talk about Avatar and 3D in one breath, but Avatar was a beautiful film full of rich computer-animated world design with or without 3D, and I never saw anything that the 3D element added to the film. Anyway, the 3D usage in Immortals is terrible. There's 3D lens flare that hovers in front of characters; there are fight scenes where the depth added by the second camera somehow seems to suck dimension out of the fight, rather than add it; and there are scenes where a combination of narrow depth of field and 3D combined so poorly that I gave up and just removed my 3D glasses.
On the positive side, Mickey Rourke does an excellent job with limited material. His character is supposed to be a ruthless king, bent on the destruction of the gods, and he plays that up well, as long as he's not actually reciting any of the unimaginative dialog he's been handed. John Hurt is credibly wise in his role as Theseus's teacher. The rest of the cast, I think, don't have enough meat in their roles to really get any traction. No one is actively terrible, but it's hard to tell how good they could have been.
So, bottom line: don't see movies in Revere, MA and don't bother seeing this one at all. Zach Snyder is currently writing a sequel to 300 and directing a Superman movie. If you enjoyed 300, wait for those. If you just wanted to see great sets and lots of ass-kicking, go watch Thor again.