Finished the main game today too (on PS4) : I must say, this is one of the best games I played during the last few years.
Not only does it make an excellent showing as a puzzle game (looks simple at first, but the design keeps astonishing you and pushes often enough a "think outside of the box" mindset that led to a few "eureka" moments for me) but it also does an excellent job with it's scenario and writting.
The "philosophical" part is far from being pompous or over-bearing, it's actually cleverly distilled through the game, documented and, while not revolutionnary in it's topic, drives your mind to think of the problematic outside of the game context. I was even pleansantly surprised that the game does not hesitate to challenge you on your interpretation/stance on the topic without adopting a moralizing attitude (well...in a way...but that's part of the plan).
I really liked the music as well. It has it's charms graphically but it's not the most striking art direction either.
It comes high on my puzzle game list, probably on par (if not better) than the likes of Antichamber, Portal 2 and The Swapper (whose writer...also wrote for Talos bytw).
Good job Croteam, you got me out of left wing there, never expected this kind of product from the "Serious Sam house" but I'd surely encourage you in that direction.
A must play, it would have been in my GoTY list if I had played it earlier, and rather close to the top 3.
I won't start the DLC yet to avoid burning out on it but I'm really looking forward to it already.