I bought it on release but my **** poor PC and unoptimised game engine meant I had a pretty patchy experience. Mate of mine who I told to pick up Arma 2 this weekend and who is a big FPS fan. Played at a fairly decent level of CS plus the top of the DOD scene when it was popular. He also plays Quake 3 still.
Anyway he tried the free weekend and says he has put in 27 hours between late Sat-Sun. Urging me to buy it not knowing that I already own it. Will need to re-install and see the changes for myself.
It's definitely worth a second look. Also, there has been a big increase in the amount of custom maps, some of which are fantastic, and mostly available through steam workshop.