I picked this up in one of the tru b2g1 deals, expecting to play it in a weekend and then trade it in while the value was still high. But once I finished it I found that I didn't want to trade it. I almost never replay games, but I can really see myself replaying this one - its short length is actually sort of a plus in that regard. The gameplay is fun - its just a tiny step down in quality from God of War, which means its only excellent instead of incredible (though the Kai shooting levels take a bit of getting used to, they get much better later in the game when you're sniping people from a longer distance.) The cut scene work and story are surprisingly engaging and effecting - Narito is a great, really strong character. I was also underwhelmed by the demo, but I'm really glad I picked it up anyway - I was really surprised by how much I enjoyed this game.
BUT - if you're the kind of gamer who skips cutscenes and plows through dialogue to get back to the action, this may not be the game for you - the great storytelling is as important as the gameplay. The length of the game is really what's necessary to tell the story - in my view easily worth the $40. I wouldn't have regretted paying full price honestly (glad I didn't tho!).