Same here. Still in 1st week. Went from 4s after almost 5 years. The 6s feels full of power and very responsive.
I think that would be 4 years on the 4S, since it came out in October 2011.
And I did the same to the 6s. The only slight negative was comparing the screen to my friend's Galaxy S5. Fonts on the S5 are noticeably more crisp, as I'm sure they are on S6 as well. Still, I prefer the 6s to even Samsung's latest and greatest for the Apple ecosystem.