The first 5 maps are completed for the Steam version of Black Mesa and can be downloaded here:
Make sure to also download the resource pack that contains custom materials and sound:
I split them in two to make it easier to update the maps if I needed to fix them and vice versa for the resource pack.
To sum up the maps, think of Serious Sam meets No One Lives Forever 2 and Black Mesa.
Lots of battles, more enemies you normally meet, a weird sense of humor and Black Mesa styled environments.
So be in a happy mood and go pew pew some of them evil folks.