It was shorter then I expected, but a great game non the less. 6 hour was my time for the main story I was really playing through it lesurily though. If you're looking for a game with a good story, buy this game. If you're looking for a game with smooth controls and well thought out levels, buy this game. If you're looking for a game that you can sink many hours into, don't buy this game.
TDLR: Great platformer, good graphics, runs well, very short.
This game is beautiful, with awesome vistas. The music is pretty good, the story is a bit bland. Most of the time the platforming is decent, but some seriously bad mechanics and annoying controls make this a frustrating game in single player. Only play this game in co-op, its not worth doing the repetitive levels over and over again.
Interesting puzzles and good atmosphere make for a pleasant experience. I can't say too much about the story since I just couldn't get into it. The 'leveling' seems a bit slapped on and doesn't really add too much since you're effectively railroaded into it anyway; same could be said about the inventory. These things and the occasional focus on fighting things just don't seem to fit very well. Some skellie killing is all well and good, throwing a boss into a puzzle room is much fun, but the times when you're screen locked into fighting 1by1 spawning is weird for this kind of game.
As for the co-op... Honestly it's way easier to do yourself. And the level doesn't change with multiple people so it's completely unnecessary to have the other person. If anything you're just slowing each other down. I would not recommend this as a good co-op for those looking for a challenging puzzle game to play together. For just horsing around, sure.
Trine is not a bad game, but I wouldn't list it as the most amazing/best platformer of its time. It's interesting, a little different, and could be something to do for a little while.