The problem with FS is that since it's a mod, there's no way to download a demo for it or anything. You have to pay up for BF 2142 and then download FS, and if you don't like it, you've wasted your money.
I personally felt it was money well spent, though. I've played some hours of FS and not a single second of Vanilla BF 2142. Nor do I plan to play Vanilla any time soon.