Well, as a commuter on Biggleswade to King's Cross.
FCC GN Route are a mixed bag.
On the plus side, they've done a lot of refurbishment work - far more than WAGN did - on stations (particularly reopening waiting rooms) and in the trains. Some has been good, some very perfunctory. Cleaning - particularly evenings and weekends - is significantly better. Timekeeping, generally, is rather good. Clever steps have been taken (cascaded trains, stopping patterns, providing viable alternative journeys) to reduce overcrowding based on decent passenger survey numbers. Information at times of disruption has improved vastly since WAGN, even if it did take them a while to get this together. Perhaps controversially, the evening peak restrictions are great for commuters, if rather less good for day trippers. The new Super Off-Peak ticket, though, is a genuine reduction, and very well priced.
On the negative side? More 321s on the Peterborough Route, and these are incredibly uncomfortable trains - it's almost better to stand. The unrefurbished units that are still kicking around are diabolical. A small but visible section of the RPI staff is aggressive, rude, and acts illegally. Some gateline staff can be like this too. Restrictions on folding bikes were changed, but by redefining terms, and generally unannounced. Fares are a joke - this is hardly unique to them, but they're definitely not at the lower end of London commuter fares. First Class is terrible, and so badly policed I'll often avoid using it if I have, say, a connecting FC ticket via Peterborough. The current safety announcement & departure board fiasco is just silly (so glad I have the iPhone app).
I've had worse, but the annoying thing is that so many of the things going wrong are down to a bad attitude.