I was reluctantly blown away.
After downloading the demo, immediately getting hooked, buying the game, playing it for some 8 hours straight...
I must say that this game lives up to all the hype.
I would say it's like Mario, but there are only a couple similarities.
MARIO?
1. It's a side-scrolling platformer. But really, must all side-scrolling platformers necessarily be compared to mario? This is more of a side-scrolling platformer puzzle game - less action, more puzzle, but the action is still there.
2. Mario references. This game is full of them. It's like an homage to Mario games, but such a good game on its own merits that it shouldn't be seen as a homage to mario games.
Regardless of its similarities to old Mario games, this is a masterpiece of its own.
Story - 10 - Top notch. Excellent writing, interesting storyline, fun references, original interface for telling the story.
Sound - 10 - Amazing. Beautiful classic style music, fit right in with the graphics, different depending on the pace of the round...
Graphics - 10 - Beautiful. 2d art at its current best. The graphics suck you into this new world.
Controls - 10 - Intuitive. After one day of serious playing, these controls feel instinctive. I had no problems from the first time I picked it up, to the last 2 hour marathon of trying to place first in one of the speed runs (I ended up in the top 30, was great fun).
Gameplay - 10 - Original. The only thing here not completely unique is the 2d platform. Every round sucked me in. It was like some game designer had saved up awesome ideas for years and threw them all into one game - this isn't the kind of gameplay you get with a rushed, business $$$ based game. This is art.
Value (aka Final Score) - 10 - This kind of game doesn't come around often. You can feel the love put into it, and the insight behind it. It's ridiculous you can experience this for only $15. I can't let myself give this less than a 10.
If you only own 2 xbox live arcade games ever, own this and geometry wars 2. I'd recommend n+ as a close third though, for sure.