If ANYONE is planning on seeing the movie, I would just say the books are much better, in comparison of course.
For the most part I understand they, of course, weren't as big a hit as Harry Potter so they didn't have a budget for all five books. So looking at it as ONE MOVIE its got a solid B for itself. If you want better though I highly reccomend the books.
The following is my complaints about what they had to take liberties with since they were limited to one movie. If you've read the books you'll know what I'm talking about, and if you're going to see the movie this'll spoil the whole for you (if you care of course)
+ Show Spoiler [ranting] +
First off they tell you he's the son of Poseidon, dont even BOTHER with all the work they put into it in the book. It starts off with the cliche but well done 'kid with problems'
who's 'different' but of course you don't know WHY. Given that they had to make the previews exciting you kinda lose out on the whole WOAH HES THE SON OF POSEIDON. Next is how they handeled the whole quest, first off its NOTHING like the quests in the book they could've at LEAST put the Oracle in there. So basically they shaved off EVERYTHING about kronos and tartarus even though they waste time mentioning him in the beginning of the movie. So onto to the quest, its to get the Pearls, these pearls are what Percy gets from a sea creature when he visits I think its the famous arch in st. loius? He never goes to the recreation of the Greek place in nashville. Luke in himself seemed like a total joke character without the books way of hiding him as the bad guy until the end. First off they give you no OTHER option for someone to betray Percy no do they hint at it, so once you meet Luke you automatically can click oh wow he's the bad guy. I TOTALLY SKIPPED THIS, when he's at school Grover gets picked on by some random chick and he gets mad and splashes a ton of water one her which is WHY MRS.DODDS asks to talk to him. Not because he just told James Bond(centaur dude) his name. Next you have the whole going to camp thing, in the movie you have 18/22 year olds playing SUPPOSEDLY TWELVE YEAR OLDS theyre supposed ot be twelve because the whole books plot revolves around when they turn 16 there is a whole prophecy for that. Since they scrapped everyone involved in the four other books that too got kicked to the curb. Now we're goin to touch on the camp where once again, anything that hints even a LITTLE at the plot from the book gets tossed. He was supposed to still be 'unknown' when he gets to camp and is tossed in with Hermes' cabin until the capture the flag where he fights another missing in action person Clarise or w/e who is the daughter of ares. She hates him because he killed the minotaur which after he beats it and gets the horn, they just toss that way too. So he fights HER not ANNABETH in capture the flag, and he isn't the awesome hero of CTF either, what happens is he's on the same team with annabeth and Luke. Clarise earlier wanted to get revenge on him for disgracing her somehow by beating the minotaur and he splashed her with toilet water. So she runs off in the CTF to beat him up, this all being part of Annabeth's strategy to keep her out of the fight so they can win. THATS when its shown hes the sun of the poseidon and they have the whole him in the water and healing and shit. ALSO WTF HE CANT HEAL PEOPLE WITH HIS WATER WHAT IS THAT WHAT IS THAT, COME ON SERIOUSLY? YOU CAN TAKE LIBERTIES BUT THATS NOT HOW IT WORKS. IT HEALS HIM BECAUSE HES BASICALLY AQUAMAN ITS NOT AN ELIXIR OF LIFE, so terrible. The fight with medusa, what happens is he's on the bus, he fights some of the Furies (There are more than Mrs.Dodds) and he gets kicked off the bus loses his food and him with Annabeth & Grover find a food shack place. But instead of a run down statue weird garden, its just a nice lady who gives them snacks GROVERS UNCLE NEVER WAS 'KILLED BY MEDUSA' he was out on his quest for Pan god the wild and was THEN killed but you are not told that. Grove simply says my Uncle went missing not OH SHIT KILLED BY MEDUSA GUYS GOTTA GET OUT. there was no OTHER LADY there, but I guess they just wanted to show off their human to stone CG powers.
THEN GROVER CUTS OFF THE LADYS ARM TO FREE ANNABETH ROFL, YOU DO KNOW YOU CAN REMOVE PETRIFICATION RIGHT? NO STICKY GLUE IS GOING TO FIX THAT. But thats just being picky it was a minor character, but this isnt being picky this is just ridiculous. HE DOESNT FIGHT HER WITH AN IPHONE TO USE AS A REFLECTION, I KNOW THAT ITS 2010 BUT SERIOUSLY?! SERIOUSLY?!?! AN IPHONE?! COME ON!! Finally theres the whole deal with Lotus casino IT PUTS THEM ON A DRUG TRIP HAHAHAA, ORIGINALLY THEY GET THE CARDS AND CHILL and when they casually are making conversation with someone they realize oh shit this is the time fun zone and they leave, NO PEARL, they just stopped ther because they were out of food/shelther. Which btw they never touch on ambrosia at all, so silly its not like id ruin the plot if they did. Yeah im tired of ranting theres plenty more plot changes like when they use the pearls his MOM STAYS BEHIIND, and Hades frees her because hes NOT the bad guy and a long with the Lightnign bolt his helm of invisibility was stolen. ALSO WHERES ANNABETHS MAGIC ITEM? So dumb.
tl;dr I rant about all the things that werent in the movie, or changed because they couldn't afford to make 5 movies so they took the first and chopped it up to little pieces and duct-taped it together for some free money.