I just voted it as an essential title today (though I've got to admit I'm voting ALL of my games in there...as such, I'm not sure if I'm just too happy with my games).
What to say about it? Well...it's nice looking for a start (see also: pictures).
It has a leveling system where every level gives you an extra point to spend on abilities on the three characters you have. You can switch these abilities around as you go, which is probably intended.
The gameplay...it took some getting used to for me. With all that visual fluff, it was sometimes hard to correctly calculate a jump, shooting the grappling hook or arrows can be bothersome (how do you even manage to do that when playing co-op, btw?) and I have no idea how tell the warrior to make a jump while holding his shield above his head (I think I need two right thumbs for that: one to hold the button of the shield above him and another to press the jump key).
However, after an initial "normal" walkthrough and basic levels, the puzzles start to shine. Using portals, levers, pipes...there's a lot of depth in there. This can sometimes get frustrating, but it makes it all the more gratifying if you manage to actually SOLVE the puzzle.
The story is also well-written. You've got three different heroes who interact nicely (kinda like in Lost Vikings) and really something to pursue.
So...yeah. Even at full price, it's worth every penny. If you can get it at reduced price...go for it even more.
