It's an interesting game. It's brute-hard, in the sense that the game depends on your flawless execution of precision jumps, often timed.
At the same time, it's not very hard in that I bounced through the thing in 2.5 hours, and the longest I spent hung up on a room was around 5 minutes (a room universally ranked as a noticeable difficulty spike, I might add). Most of the hardest jumps in the game you can clear in about a minute or two because Cavanaugh, the Irish designer of the game, gives you basically instant respawns near everything tricky.
So, I died 1122 times, and made about 4200 inversions (jumps, more or less). I died 1 in 4 times I jumped, and over 7 times a minute.
That's ridiculous, except that (from forum posts I found) I did about average. Cavanaugh reduced the cost for death to be so low as to not really matter, and although you thought, "I hate you, Terry" for a few traversals, it never really made me lose focus on the game.
My least-favorite feature was probably the floaty slowdown. Your character has just a touch of slowdown when you let go of the keys, and although you adjust to it, you get a lot of crap deaths sliding into a pit of spikes because you didn't quite let go at the right moment. However, as noted above, who cares?
It's not the game for everyone. It's hard, doubled by the number-pad control. Although I loved the 8-bit sensibility and the artistic use of color and shape, a lot of people don't get nostalgia seeing something that simple.
It's worth playing the demo, though, to see what he hath wrought.