I remember the idea of "first to file" (FTF) being explained to me in the past, although I don't rember where or whom.
Basically, FTF doesn't trump prior art. If someone invents something before you patent it, the patent is invalid. This doesn't change.
One of the big litigation problems with "first to invent" (FTI) over FTF is that, when two companies claim the rights to have invented something first, it take a HUGE amount of digging, research, and legal discovery to figure out. Especially when companies keep secrets and have long R&D periods; it's a tangled mess to figure out the exact timeline that grants patent ownership to one company or another. With FTF, you don't have this problem, because its blatantly obvious who filed first, and prior art is easier to prove in court.
IANAL, so correct me if I'm wrong!