I just downloaded it yesterday actually and played for 2 hours and it's really fun.
I'll probably get bored of it in a few days but I'm having a good time so far, probably b/c I'm in the newbie server and getting lots of kills.
It's easy to find a room with ppl playing and I haven't experienced any lag (latency) problems so far.
There's quests and acheivements in the game, which kind of gives you a purpose to play.

I haven't played much other free fps's so I can't really compare them.