At no point did I ever find myself trusting Sylens, even when I was glad for his help. That scene at the end felt like vindication of that attitude. I still don't know what exactly that dude's endgame is, but I'm fairly certain it's bad freaking news.
More generally, I love the game. I really like Aloy as a character and I found the world and the backstory genuinely compelling. The gradual revelation of the mystery did manage to keep me guessing most of the way through even if there were a few bits which were fairly obvious earlier on (like Aloy being essentially a clone).
I liked a lot of the stuff in the environmental storytelling too, and there were tiny stories told in some of that which elicited strong responses from me (at one point I recall an audio log essentially having a character gleefully rejoice in the ability to play god and finding it genuinely unpleasant to hear).
As for the way that stuff was presented on the other hand...Not a huge fan. I don't really like audio logs and in-game compendia of text snippets at the best of times because it feels little less disruptive than having to look things up on a Wiki but the audio logs here especially stand out since they have the same flaw as those MGSV cassette tapes in that the game presents the option of listening to them while you play...Then ignores the fact you are so you miss out on audio cues and narrative dialogue. Not enough to detract from the game much, but a frustrating niggle, for sure.