You asked me if I'd actually played it - an insult
I'm sorry but I didn't mean that as an insult. I meant that as an honest question.
But ok, here's why I think it's a great FPS:
It has varied and deep movement system - strafe jumping (and bunny hoping in CPMA), rocket/nade jumping, plasma climbing (that one is not really suited to the deathmatch gametypes, more useful in Defrag, but still a nice thing)...
A few but very distinct weapons each having their specific use.
Good maps which complement the movement nicely + editor (and therefore lots of great community maps).
Modability (CPMA, Defrag, Urban Terror, Tremulous).
And then, it's not just about good aim. It's about item control/timing, using the right weapons at the right time, listening to the enemy pickups, knowing the spawn points, predicting what the enemy will do next.