Immediately - I love it. The way that your play area gets "lost" and you suddenly feel that you're in a massive expanse is fantastic.
The graphics are certainly decent enough to carry off the concept very well.
It's very easy to pick up and play without a massive learning curve. Love that aspect to it.
Would I buy it? Definitely.
Ok, thoughts on where I could see improvements.
Only bug I've seen is the the area you get to after you've had the 2 wood rods unlock the area you can get into by simply standing on the sliding rock that takes you forward then walk through the wall. Make that stone that takes you forward only appear once the unlock has happened.
Shadow - it desperately needs some kind of shadow, a body shaped one would be great but even just a blob would make a massive difference to making you feel that you're actually there.
Sound - this is the area that needs the most improvement imo. Heavy sounds of sliding blocks, sounds in your ears to put you off, the sound of wind, step sounds when you get onto the blocks etc.
Ways you could improve things - I admit I've not completed the demo so some of this may come in later, but these are just my thoughts on what I think would be good additions.
More variation - different sized blocks, blocks that break away in part or after a certain time, blocks that teleport you around, glass blocks, slippery blocks, blocks that go in a pirate ship type swinging motion, that kind of thing.
More height - play on the being high up thing more, vertigo is great!
Things that fly at you to put you off what you're doing.
How about rooms that are in the dark and you have to do things being guided by a voice in your ear ?
Walls you can break down in some way.
Opening and closing metal doors with teeth, prince of persia style.
There's so much scope, but this is just spitballing. I love the game as is and cant wait to see where it goes.