Genuinely one of the only fan games in the valve universe to actually feel like it fits perfectly, the writing is definitely good and up there- something that honestly could compete with modern AAA writing (albeit not hard but it was a small Dev team).
Aiden walker himself is a character I really love, I think that he has corny-ish and edgy lines at times but I think in a way that's what makes me like him even more, although the only nitpicking I have is that he feels a tad bit mary-sue at times but in a way it doesn't really matter since every other protagonist in half life has the same trait.
The angle and new view of being the 'bad guys' of half life was actually really fun, and personally I enjoyed it a lot more than the hl2 base game with Gordon, and I really love half life opposing force as well so it was a perfect game for me. Wilson was such a good character too, and having to make that conscious effort to haul him around and slowly see Aiden's tough guy personality fade away around him- (especially considering his backstory) was probably the icing on the cake for me. I cried like a bitch near the end when the whole Wilson death scare happened, and I think I genuinely got very attached to having him around 24/7.
All the endings are decently bleak and honestly, before I did the gman ending I guessed that it would just be his fate to have a bleak and empty ending, which disappointed me slightly, but after the G-mans ending and what I consider the true ending it left some hope that there's going to be a continuation of entropy zero and overall was such a fitting way to end it.
I highly recommend this fan game, not only is it free if you own half life 2, but it was probably some of the most enjoyable and fitting gameplay and story I've ever seen for a fan game.