I spent 7.49 Euro on Project Temporality, and I'd honestly say it was worth it. The game is short, though. The game runs about as long as the original portal and has more than enough puzzles to be interesting, with some of them requiring very abstract thinking. It looks pretty good technically, though it's very samey, and I wouldn't with the story if I were you. But should you get it? Well...
If you like puzzle games, DEFINITELY buy this. It's one of the better puzzle games I've played in a while and - unlike the others - it doesn't look awful. The controls are loose and wobbly, but with essentially infinite-use time reversal, you can account for that. It's clever, it's creative and it gives you mechanics which seem familiar, but are fairly unique on their own. This will challenge your brain, but in a good way.
However, Temporality is also a very slow-paced game. While you may beat most levels in under three minutes on the clock, keep in mind that the clock counts back when you reverse time. So while it may be an absolute 3 minutes, you'll actually be spending 15-20 minutes per level. The character jogs slowly, jumps awkwardly and the game requires a lot of repetition, minor adjustments to positioning and timing and a lot of staring at a puzzle to figure out what you to do. If you need action, this isn't the right game. And it also goes without saying that if you're awful at puzzle games, Temporality will frustrate you.
For me, though, it was a worthwhile buy at 7.49 Euro. I'm not entirely sure if I can recommend it at its full price of 15 Euro, though. I mean, it's cool and creative, but also VERY short. If you're a huge puzzle game fan I can kind of see paying full price, but for my pocket I don't know that I could go much higher than 10 Euro. Then again, I'm stingy :)