Having not played TF2 my opinion is a little biased.
I played TFC up until the point where [DW] disbanded (if anyone can remember) and then moved on.
FF is a very good remake of the game, maybe a little too close since for the prem clan's its a swift move over without any learning curve, which for new players is far to steep when you log into a Noobie server and see b'hop conc'ing spamming medics it's kinda confusing.
They should have changed a few things, Conc'ing maybe not taking you as far as an example.
They did however make the spy class VERY good and great to play, also the pyro class with the stacking flames is a great change also, if not a little annoying.
The bugs are quite bad, HWG are far to powerful, SG's have too little health and the accuracy i feel is pretty poor. Too few levels to play, should have added the original levels first, then the self made modded ones like sd2, monkey etc etc
But overall it brings back the feel of TFC which as a community was really good if not a little too elitist. If the leagues start up again that'll be great. And if clans like [DW], [UA] SirBruce solo'ing everyone

(if anyone remembers them

) come back, it would make the game enter a whole new level of activity.
To the new people who have never played TFC original before, give FF more a chance before you dismiss it, it requires a whole new level of skill (like said before) to play and the satisfaction of doing your first conc from 1 side of Well to the other and flying through the air is very satisfying.
To the people who came from TFC to TF2 all i have to say is if you prefer a lesser skilled slower game at your old age, then senility will take you!!!!