A very good re-imagining of the Half-life 1 gameplay! All the adjustments, details, new music and graphics simply rock!
The source engine is really put to good use and drained of all its capabilities. In many parts, "Black Mesa" is even better than a purchase-able game (yep, it is totally free!)
I'm also playing it with the 3d Vision 2 and glasses and it's so immersive! All those details and realism! And the source engine boasts very high frame rates.. ok, there are some parts that are a bit poorly optimized, but it's hardly a problem. The boring parts of the game have been changed and tweaked. I'm sure you heard this a lot but it really DOES feel like a new game while also keeping some familiarity (and with very high nostalgic factor).
Some of the weapons look fantastic! The crossbow, for instance. It looks like it's for real - that is really incredible when knowing that this team are just fans and not really hardcore game designers. Looks like their passion simply makes them more talented and hardcore than any professional, full-time game developer!