The first Entropy: Zero was a fun but flawed experience. This sequel is so much polished and better paced. The level design is excellent, with interesting encounters and great art direction.
They've nailed the HL aesthetic. The HL lore is used in a very interesting way. All the creatures, items and locations seem to fit well in this universe. They've added a lot of creatures from Xen compared to the original HL2 which is fun.
Combats feel more punchy than the original game, and all the new weapon animations are great. There is also a new grenade with an interesting concept.
Puzzles are also interesting but not too difficult or repetitive/annoying to solve. (Not like the 50 cable puzzles in Black Mesa Xen for example).
The writing is also surprisingly good. At first it's a bit weird to have a talking protagonist in a HL experience, but the dialogue between 3650 and Wilson is fun and touching. 3650 seems like a standard asshole character at first, but we keep learning about his struggle and feelings towards the adventure. The voice acting is also pretty good, I was surprised that it was not actors but the developers themselves.
Took me around 7 hours to finish. It even has a great soundtrack and achievements. It's mind-blowing that it's free.