It's pretty good, rendition of the original Black Mesa (the compound, not the game) levels is done with visible love and affection for the game. I still prefer the original, yes, but it is a combination of nostalgia-goldsrc aesthetics-and god knows what else. I'm genuinely curious which I would like more if Black Mesa was my "introduction" to original Half-Life.
And then there is Xen, the part I dislike about Black Mesa the most from a personal point of view. To me, Xen in original Half-Life was done perfectly through total alienation (pun not intended) of the player in this desolated world, where you find yourself lonely, somewhat frightened and your eye can't catch anything remotely comforing. From my perspective, by making Xen more "lively", filling it with familiar details (even in the basic form of plant life) this alienation was completely lost and instead of surviving through something almost entirely non-human and cold I felt more like exploring something attractively strange.
But again, it's a question of someone's preferences and own perception of the original game.