Prior Art still applies.
Even with the 'First to File' system, a patent can still be overturned if there is public evidence of 'prior art'.
If you invent a new kind of bicycle and ride around on it then, some time later a Cycle manufacturer patents the same kind of bicycle, that patent will, almost certainly be overturned due to prior art.
If, however, you had the idea for the new bicycle, scribbled it on a piece of paper and stuck it in a drawer, then there would be no publicly verifiable evidence of prior art and the patent would stand.
The 'Windsurfer' case is the classic example:-
The fact that there was physical and public evidence of prior art blew the patent away.
Its also the application date of the patent that is important, not the date it is actually awarded. If you come up with a new idea and take it to some company to see if they would like buy the idea and demo it to their MD, THEN decide to apply for a patent then you are in trouble. The company can start making your gizmo AND blow away your patent because the minutes of the meeting where you demoed your gadget pre-date the patent application. If you had applied for the patent beforehand then your patent should be safe.
The flip side is that if the Gizmo company decides to do the dirty and patent your invention themselves your evidence of prior art will shoot down their patent. If you were sensible and applied for your patent before making your invention public then your patent will invalidate theirs as your application date was earlier (even if they managed to get the actual patent awarded before yours).
Though this all does, to a large extent, depend on who can afford the best lawyers......
NB, this is all from my memory of UK IP law. I haven't dabbled in it for 4 or 5 years so all of this may now be completely bogus......