If you haven't played either FTB1 or 2, just get FTB2. If you have played FTB2 already, wait until FTB3.
Having played FTB2 for almost three years, I thought I'd try out FTB1 as well. I can't really review this game fairly because I had already played FTB2 and I know this game isn't as good as the second.
Good: every level/mission has a different environment and setting, fun campaign, good online multiplayer
Bad: squad AI interferes slightly with game play (I failed a couple of missions because of Lonestar interfering and dieing, but he's still a great help throughout the game), MANY cheesy lines in voice acting, campaign gets repetitive after the third or fourth mission (e.g. infiltrate, obtain intel, capture so-and-so, extract), not many weapons to choose from
Online: best part of game, many maps, competitive, fun
All in All: Basically, both FTB1 and FTB2 are identical except FTB2 has about ten times better graphics, cleaned up controls, more weapons, a slightly better campaign, and of course, the FTB2 online community is more active. If you haven't played either game, just get FTB2. If you have played FTB2 already, wait until FTB3.