I first played the demo months ago and instantly fell in love with this game.
The full game did not fail to deliver.
The whole concept of (literally) standing on the shoulders of those who came before is presented wonderfully. See a deathtrap? Die on it, then respawn and walk on your corpse. Switch that has to be held down? Die so that your corpse falls on it, then respawn and walk past. Et cetera, et cetera.
The controls are a bit inaccurate at times, and will most likely leave you dying slightly more often than intended. Although since this game is all about sacrificing one knight to make a path for another, a few more corpses lying around aren't that much of a problem. (And if you're finding it difficult, a few more corpses aren't a problem at all.)
The puzzles are challenging but not frustrating, and make great use of switches, platforms, corpses, and checkpoints. The solution is always there, if you know where to look.
The music is unintrusive, yet amazing. It fits perfectly into the game. The sound design, too, is brilliant. Everything just works together really well.
There is tremendous replay value in this game, too. Times (and deaths) to beat, secrets to find, more knights to sacrifice... And if you want a quick game (or want to prune Jim's family tree further), there are no lengthy menus to keep you from killing knights. This is evident even in the demo. Ten minutes worth of content, and I had over an hour logged.
There's also a great (if morbid) sense of humour present. The knights' death cries are hilarious, the names are generally overblown (and if they're not, it's funnier), and the victory screens... well, "VICTORY is failure rounded up!" is just one of the possible messages to recieve upon finishing a level. And then there's the credits of the demo.
If you haven't played the demo yet, you should. Even if you know you want this game. The credits alone are worth it. Possibly the title screen as well. And if you're undecided, the demo is basically a slightly earlier version with less levels and slightly different graphics (mostly just checkpoints and the victory screen are different, really).