I used to play TFC constantly back when it was more popular. I was always an engineer, a regular defense mastermind - between me and my sentry gun (and the occasional dispenser detonation), I was the lone guy who always stayed back and watched the flag rather than go on offense like every other guy. Those were the good ol' days.
These days, Counterstrike Source occupies most of my gaming time. TFC just feels like a dinosaur to play through, and with neither TF2 nor Fortress Forever looking to be released soon I'm stuck with wondering which one will ultimately take the cake. Some people tend to take a negative opinion of TF2 based on the little I'm sure we've all seen, but I'm going to reserve my final judgment until both games have actually been released.
Sure, Fortress Forever looks more like the TFC of old that I'm sure we all remember, but for all we know the new improvements the developers are promising could turn out to be totally awful and ruin the thing. The same goes for TF2. There might be some new twists on the gameplay that really add fun to the game despite the Toy Story-esque graphics, or the gameplay might be as silly as the graphics suggest. TF2 has "official" support in its corner, while Fortress Forever likely has the backing of the majority of the original TFC community.
I do think that one will eventually snuff the other out, but it's hard to say which one.