If you haven't seen the show, stay away, if you'e a fan of Lost, you don't want this game to be your constant.
You play as Elliott Maslow, a character made just for this game, who is a photo journalist trying to recover his memory while stranded on an island. Your primary objective in the game is recovering your memory, which you do by taking photos in flashbacks (something all Lost fans are familiar with). This is an interesting idea, but taking photos can be a real pain at times, and you may constantly have to retake the photos to get the right one.
In flashbacks, other than taking photos, you can have conversations with select people, read newspaper clippings, listen to audio tapes, and a few other things. These things don't really relate to the main story at all, but fans of the show may pick up a few things a long the way.
On the island you will explore, do a few missions, and talk to characters from the show. Fans of the show will enjoy visiting the various stations in the game, like The Pearl, The Staff, and of course The Swan. These locations seem to stay true to the show in pretty much every way. Unfortunately, the game only takes place in the first two seasons so you miss out on a few locations, like The Barracks.
The missions on the island are boring for the most part, and include running from the smoke monster, searching for dynamite, and going through caves. None of these are particularly enjoyable. Running from the smoke monster is cool at first, but becomes tedious later on. You have to run through the forest following flags or tree marks and hide in trees when the monster is too close. When you leave the tree you'll have to get back on track, which can be hard because of the way the tree puts you out. Going through caves is borderline awful, as it's too dark to see a hole a few feet in front of you, which will kill you if you fall in it. You'll have different light sources which will be put out by bats or waterfalls. Without light you will die.
About half of the characters from the first two seasons appear, and even fewer have a meaningful role. Those who do have a role seem to actually be parodies of the characters. Jack will give orders and get in your way, Kate will be secretive, Sawyer will sound like a generic redneck and give you a new nickname every time you ask him something, Michael will yell for Walt a lot, Locke will sound wise and not want to leave the island, etc. Some characters like Claire will only appear twice. The entire tail section, Boone, and Shannon aren't seen at all. The characters will either look like the actual actors, or nothing like them at all, there's not really anything in between, and it's a shame that they couldn't get all of the actors to do the actual voices for the characters, because a lot of them sound terrible. Even with the flaws, it's somewhat fun to interact with them.
The environment graphics look fine for the most part, but the jungles look nowhere near as good as they do in GRAW 2. Characters, as mentioned above, either look good or awful. The ocean does look quite good though as you look off hopelessly on the beach.
The story has a lot of things from the show missing from it, and other things seem to come and go too fast. I guess that's what happens when you try to fit a huge show like Lost is a relatively small game like this. Sawyer, Michael, Jin, and Walt going on the raft (and the subsequent kidnapping of Walt) are only mentioned once in a passing conversation, making it seem as if it wasn't a major part of the show. The hatch being found is made to seem like a small detail. It's pretty much like "Blah, blah, blah, oh, and we found a big hatch out in the jungle, thought you may want to know that".
Part of the story which would only be understood by fans of the show is the inclusion of The Others. It's handled so awfully that I can't imagine what someone who never saw the show would think. You would really have no idea what's going on at all.
The story wraps up with you taking a role similar to what Michael did at the end of season two, with a few things being rewritten. The end of the game is actually interesting and is something new that hasn't been seen in Lost yet.
So overall, it's a fine rental if you're a fan of Lost or need some easy gamerpoints. There is no replay value at all, so you definitely shouldn't buy it. If you plan on playing the game to see if anything is revealed about the series, don't worry about it, nothing is. The game isn't canon to the show so anything that happens in the game will not have any meaning on the show.