Actually three, Dragon, the BM server and a little help form Iceman made me join.
I like blackmesa because it feels the most class balanced map I've played so far. Pyros may dominate the vent system, and there is many corners for them to be behind, but if you're a soldier you can launch them in the air to get those flames away from you. There are plenty of evil sniper spots, but there are at least 2 or 3 ways to get behind any sniper on that map. Spies get a playground of corners and hallways to make their much needed emergency exits. And engineers really nail down the "Area denial" part of the job description down...
I have become so used to meeting up with all 9 classes on blackmesa that I was a bit surprised when I joined some random servers and found half of each team to be demoman, with the occasional pyro and engineer.
Anyway, those are my opinions I've generated by playing there (almost nonstop).