From the visual and audio aspects it is an amazing mod.
However, what they failed at is how to tell a story: that's where you see Valve's experience at mixing shooting with slow paced moments to relax the player after a fight, you can see Valve's talent at invisibly guiding the player so that it would listen to the engineers around and get the story, and lots of other things.
The people behind Black Mesa failed here, they chopped the original game in lots of places which makes the story hard to understand and at some points, I found that the directions are hard to spot or missing completely - the player might not know what to do, where to go, might get confused if he/she didn't play the original game.
They also failed to see how some scenes in the original game were not there just to make the game longer but actually to make the player more emotional, to care about the engineers and cops in the game. They butchered those moments (for example the engineer "hiding" on top of the generator that got electrocuted in the original game, now he has no worries in a small room on the side of the generator).
In other places there are engineers hiding or panicked and they removed them thinking they were pointless but they actually had an important role, to teach new players how to crouch or do long jumps or introduce them to electrified water and other dangers.
Some parts are very shrunk, for example the part where the soldiers are coming to "rescue" them to the point the player doesn't even get a chance to be afraid of them or spot their behavior, see how they throw grenades or hide behind crates.... 5-8 soldiers dead and that scene is finished.