I didn't have high hopes for this, and I must say I'm impressed.
Black mesa is a step up from Half-Life in almost every way possible, but particularly fixing a lot of confusing level design and pacing issues the original Half-Life has, particularly with the new XEN, which even as a person that liked the original XEN, I must say that this one is much better designed, although I prefer the aesthetics of the original one, and do think that this new version is a bit too long.
I though that Black Mesa was going to be just "Half-Life but it's actually Half-Life 2.", but it's a lot more than that. Even my fear of it being a bit too stale without the Bhop that HL1 had quickly disappeared as I discovered that this game has a really fun "slide" mechanic, which basically allows you to conserve a lot of speed if you crouch after jumping.

Generally just a very good remake, even while I'm opposed to the idea of remakes.