I'm gonna try it once, if i pass from there one of these days.

The three popular ones i know of are that, shake shack and in'n'out (which is unlikely to open anywhere in Europe anyway).
We don't have Shake Shack here in Italy, but there's a "good enough copy" (at least i'm told so, from people who had it in London) called "Burgez" that i tried, and that was pretty good i thought.
So i'm curious about Five Guys now, since it was one of the other burger joints i saw mentioned a lot.