I've played it until level 18 yesterday (Didn't quit for any reason related to the game) and liked it a lot. The levels all seemed appropriately challenging to me and they were all quite fun.
One of my concerns was that the creepy atmosphere I felt in the beginning didn't really last, as it stayed very constant and therefore I focused a lot more on the puzzle aspect and the atmosphere lost quite a bit of its importance to me. This one "mirror" with the sound effect creeped me out a bit, even after I've experienced it once, so I'm sure that more could be done in that regard, for example through adding more atmospheric art and especially sound as the player progresses.
Also, I felt that the "elevator"-technique was a bit overused. I find that understandable, I just wonder if there are other interesting basic techniques which could be used regularly instead of mainly that one.
Still, liked I said I really enjoyed it so far and I'll make sure to go on playing it.
And keep in mind that the above is my experience from only(?) playing to level 18, so I might be wrong or very wrong about some of it