I just got into the Shootmania beta, played it a bit tonight. The engine feels nice enough for instagib (which is the true test of a smooth/reactive engine IMO). I haven't played the other gametypes yet, but the engine is the most important thing for a competitive FPS. Maps/game modes/etc can all be added easily through modding. So far I like what I see. Still have 7 more days to check it out, though, we'll see.
I've been waiting for a really good skill-based FPS since UT2k4. Tribes Ascend is pretty nice, but I really prefer instagib, and Tribes has no modding.