So... Christopher Columbus directing. Based on a popular series of childrens/teen fantasy novels. Teenage hero, with two companions, going on a quest to stop a (kinda) magical war. Mysterious pasts, family secrets, treachery and mythical beasts. Stellar cast confined to adult, supporting roles. Harry Potter's back!
Ok, so that's something of a flippant summary, but for God's sake, even the title font is reminiscent of the Harry Potter films. However, there are several crucial differences. First, Percy isn't a wizard, he's a demi-god, son of Poseidon. Second, he has a sword, not a wand. Third, he's a bit harder than Harry. Seriously, about five hours after finding out he's a demi-god, he's taking on hordes of monsters, and wielding a sword like he was born with it.
That encapsulates my biggest problem with the film. It's so rushed. Ok, it's a quest against time, but even so. The plot covers about two weeks, of which Percy spends one week unconscious at various intervals. He then travels from one unspecified rural location to another, then to Tennesee, then Las Vegas, then Hollywood. In a week. While still finding the time to battle various gods and monsters. My knowledge of American geography is hazy, I admit, but is that really possible? Bear in mind they're not travelling via magical means. Just public transport, a stolen truck, and then a ridiculously fast car. As a friend pointed out, such things can be explained away with that faithful stand-by of "It's magic" or "A demi-god did it", but it niggled at me. And it also means there's very little time to breath, to take in any of the already slim characterisation (Percy is heroic, and just rebellious enough. His best friend Grover is loyal and interested in girls. Annabeth/token girl is feisty and good with a sword.)
The plot is similarly threadbare - macguffin 1 has gone missing, Percy is blamed, he must retrieve macguffin 2, 3 and 4, with the aid of his friends and macguffin 5 and 6 (although to be fair, 5 and 6 - a shield and winged converse respectively - are pretty awesome), so that he can recover macguffin 1. Simple. But entertaining, it has to be said. While I did feel the need for some crackers with my cheese at several points, I always enjoyed the film.
This was largely to do with the action and the design. I loved the fight with the Hydra, and the climatic battle above New York, and Percy's encounter with Medusa was genuinely creepy (although I'm petrified of snakes, so half the work was done). And the set design, especially in the Underworld, was incredible. Despite my comments about similarities at the top of the page, the Potter franchise could pick up a few tips for Deathly Hallows in this regard.
The main cast were unoffensive, but unmemorable, while the supporting cast impressed, as you might expect. Special mention must go to Steve Coogan's 'Mick Jagger look' Hades, for being particularly amusing, and Sean Bean is always good value.
In short and to summarise, good clean family fun. It won't stretch your grey matter, but it will entertain. And there's nothing wrong with that.