Visuals were nice. The lightning bolt was cooler than in the books.
Acting, ok. Dialogue, not so much. Key characters like Clarisse and Dionysus were missing.
The plot was mixed with pieces from later books. Many changes were unnecessary. The prophecy was ignored; the Big 3 had no ban on children. All the gods were forbidden from talking to their kids. This puts all the blame for the lack of contact on Zeus alone. Not having unclaimed kids in Hermes cabin (and Percy going directly to Poseidon's cabin) took away angst.
Overall it was worth seeing, and I will get it on DVD. I hope that future movies will be made, sticking to the great storyline Rick Riordan crafted!