I've been running it for a while (since the early betas) and it's been pretty solid. I've had it on my daily use iPhone 6S+ with no problems.
I haven't noticed any slowdowns on any devices ( and we pretty much one of each in the house).
Highlights for me are the new Photos, Music, and Maps. They got noticeably easier to use and more powerful. For example, with Maps it was often hard to end a route if you didn't go to the exact endpoint. Now there is a nice big END button there.
Another thing I love about the new Maps is that when you are following a route, you can have it find food, gas, coffee ALONG THE ROUTE rather near you. Before it would send you on wild goose chases way off the road. Now it shows you places coming up at the next few exits. Exactly what I want.
Finally, if you have an Apple Watch, run, don't walk, to get watchOS 3.0. It's a huge upgrade for the watch.