Antichamber

Finished Antichamber last night (well, actually it was this morning...) so can comment properly - just in case you're undecided. Did you notice I was up past midnight playing it?!

As all the other, proper, reviews say, Antichamber kind of defies description. Portal really is the closest match - it's a 3D puzzle game where you don't know what's going on half the time. But the puzzles are (mostly) completely different in approach - it's usually about figuring out what to do (or chancing on it by accident) rather than using fine mouse/keyboard skills to actually do it.

It's the puzzles where you DO need to carefully build complex constructs from coloured blocks that Antichamber does occasionally cross the line from challenging to frustrating. The controls aren't quite precise or predictable enough to match the design of the puzzles, having to restart a lengthy process right at the end through no real fault of your own is never nice.

On the whole, though, the game is outstanding. It takes an hour or two to get into - it really does challenge all of your assumptions about reality and causality as well as those about how 3D games work. Look up, look down, turn around, walk backwards, you can never assume that nothing has changed in a bizarre way.
That makes it fun, makes it challenging and ultimately worth your time and money.

I should also note it took me a LOT longer to finish than you might expect from the reviews, Reddit or HowLongToBeat.com
General consensus seems to be about 6-8 hours; it's difficult to be accurate (my Steam play-times are inaccurate due to playing offline and my tendency to leave games on pause for hours) but I reckon it took memore like 12-14 hours to find every exit of every room. Maybe I was just rubbish at it (no "maybe" about it!)...