It's Half-Life 1 but modernized - as someone who couldn't really vibe with HL1 (most likely since I played HL2 first and so it was a downgrade) I found this to be an amazing experience.
The fact that they completely revamped Xen also shows - it's the most beautiful area in the game, and while it does feel a bit slower due to a higher focus on puzzles than combat (with the last part of it featuring annoying enemy encounters too), the puzzles added in to it are kinda fun to figure out and do.
Anyone who hasn't played HL1 should take a look at this game if they want to experience a classic.