Degradingly referred to as "The next Harry Potter," Percy Jackson deserves to be judged based on its own merits. Throwing Greek gods into a teenage setting is genius and there are a lot of neat ideas flowing throughout the backstory and world building. I'm about halfway through the first book and it's been entertaining. Everyone loves Olympian gods so the combination of myths and modern stuff is appealing.
The problem was the adaptation of the book into the movie. From what I've heard, they changed quite a few things. More important than that, the pace of the movie is just terrible. The script is sullen, the dialogue isn't horrible but will be dated in about six months, and everything seems slightly less than epic.
Pieces were in place for a decent action movie. I mean, the cast was selected well, especially based on looks (Brandon T. Jackson makes a perfect goat/satyr. Sorry dude, just saying). Logan Lerman is maybe a little too good looking to be Percy but he works for the teen hot throb role. Alexandra Daddario is cute but also believably strong and a good choice for a daughter of Athena. On top of that, the special effects were good, especially the various mythical creatures, so it wasn't a budget issue.
The biggest problem in my estimation was hiring Chris Columbus to direct. "Let's hire the guy who made the first two sub-par Potter flicks!" Not a good idea. I completely blame him for this movie's failure and Columbus should have stopped directing movies after Adventures in Babysitting and Home Alone. So if you want to spend two hours wishing you could be struck down by Zeus' lightning bolt, hurry to the theater for this one.