Very fast-paced gameplay as a result of small but well designed maps. The most playable FPS using Dreamcast controller.
The single player modes are the Arcade mode and Mission mode. The Arcade mode goes through a series of deathmatch-like games, but when you kill a player you get a point and the dead player drops a coin that you (or another player) can collect for another point. The Mission mode has 45 missions that you play though in the arena levels with different objectives, like protect/free a civilian from terrorists or kill a number of enemies within the time limit. Overall, the single player modes don't add up to much play time - probably around 4 hours - but there is enough variety in the missions and good enough AI to make it worthwhile.
The US version has online multiplayer, which isn't supported in the PAL version. There is also is a 4 player split screen multiplayer. I imagine Outtrigger makes for a good multiplayer game, but I haven't tried it properly (only used it to setup matches against AI players).
Graphically, Outtrigger looks very good for a Dreamcast game. The maps may not look quite as good as Q3, but the characters are better and it always runs at a very solid framerate (unlike the Dreamcast port of UT).
Unlike UT and Q3, which aren't very playable without a keyboard and mouse (especially Q3), Outtrigger actually plays well using the gamepad once you set the control scheme to D1 (so the buttons control movement and joystick aims). This is mostly because the weapons, like the rocket launcher and photon torpedo, allow for a margin of error, but there is also an aim assist that helps.
So overall, Outtrigger is a good game and my pick of the limited selection of FPS games on Dreamcast if your haven't got a keyboard and mouse. The single player mode is very short, but fun while it lasts.