Turus got me hooked on this. Though I also partially blame Sunder for helping push me toward trying it out last night.
In short: Wow. I didn't think I'd encounter an FPS MMO that was actually fun, but the game is a blast. The missions seem to tend toward the repetitive side early on, but being as I haven't seen the post-level 30 content I don't think that's a fair assertion to make yet.
If you're familiar with Tribes and Tribes 2, you'll feel right at home with the jetpack in this game. There are some imposed limitations with regards to firing and flying at the same time depending on your hardware composition, but the in-game action is confined such that jetting around is only useful as a means of avoiding incoming fire and getting to different locations faster and without the use of stairs or ladders.
Global Agenda surprised me. It's more or less the scifi MMO I was looking for!