We tried FWC for a campaign and a few other games at the clun, but found it hard to come to a definitive conclusion on what they meant in many circumstances. i found there were concepts i liked, but not overall.
We are using Iron Cow for the next campaign. While i am not totally sold on IC it is a useful basic system which i am jazzing up with some command cards dribbed from my own WIP system for 6mm.
I am curious to try Command Horizon at some time. They seem to be quite elegant, although i dont like the (divisional level?) concept. I understand that people have succesfully customized them for 1:1 games.
There are also others out there-OHMU (Oversized Heavy Mechanized Unit) being one that doesnt seem to get a mention anymore. Pireme's "Hammers Slammers-The Crucible" also features rules for 6mm.