This is a tough film for me to review, my 2 boys (aged 9 and 10) loved the film and couldn't wait to watch it again so I am giving it a 5 as they are the target audience.
The special effects are great which gives believable bad guys for the heroes to defeat, easily.
For me the huge weakness lies in the fact they have ripped the heart out of the book, there is no character development and the important, and difficult, relationship between Percy and Tyson is pretty much brushed aside. This relationship, and how Percy struggled to come to terms with it, was one of the key themes of the book and it's removal just leaves us with a bit of an action movie.
For me, 2 stars.