Comes from Hebrew (Sof Ha'Derekh) Even though it sounds negative, it's actually very positive. It expresses the feeling after having an excellent experience of any sort. You use this expression to intensify telling stories to your friends just to make sure they understand that your experience is/was way beyond good, great or even excellent, saying it cannot possibly get any better then that. It also defines something extremely beautiful. In general it intensifies the word "Best".
someone: how was the party last night?
you: End of road!
she: so how do I look?
He: End of the road!