Hi guys. Thanks for the interesting thread.
I'm also one of the team members for the ongoing SI development, and I also spend a bit of time managing the weekly pickup games. My background (gaming-wise) is mostly in arena shooters like Quake, but I've dabbled in games like ns and tf2 as well. I played SI when it first came out a million years ago but it seems to have outlasted most of the games from its era..
At its core, SI is capture the flag but with the guns and movement mechanics from Half-Life. However, there are strong RTS elements as well. We don't have structures sprouting up around the map like in ns, but teams make decisions about what to pursue in a tech tree, which map resources to secure and so on. To me, it's a good blend of the mechanical skills of Quake-style games and the teamplay/coordination aspects of things like Counterstrike or TF2.
As Merz mentioned, we play every Sunday. Right now we're beta testing new versions of the game. We have lots of ideas for the future (namely more unique guns/mechanics, better customization support and more exciting maps). I'm very open to changing the game quite a bit, and taking it further away from just being 'ctf with tau cannons.' If you have ideas or want to give the game a try during some pickups, let us know.
Cheers!