I bought it on pre-order, played it a third of the way so far. It's fun, but not breathtaking. I didn't notice any slow-down, but I got a pretty decent PC. Given how it looks, it would be dissapointing if it didn't run properly on slower PCs. The first person jumping can be tricky, but I haven't found it completely frustrating so far. Though I'm not going for 100% completion, since it would require pretty much pixel-perfect precision on every single level or else you won't make it in either the required time, shifts or deaths.
But I do wonder why the dev left Valve to make this. I obviously don't know the inner workings of Valve, but I given how this is "Portal but with a new physics breaking puzzle mechanism", and seeing how big Portal got, I strongly doubt Valve wouldn't have let her make this game for them. And perhaps the guys who did the physics puzzles in half life 2 could've given some tips on how much first-person jumping puzzles and how you shouldn't give out the only achievements for doing 50 of them in quick succesion with no mistakes allowed. Or at least told her "Remember Xen? Not like that."