It is as slick as can be on the 5S. Everything is smooth. Battery lasted all day with constant LTE use. I've been using Google Maps navigating around all day.
There's some software glitchy-ness going on here and there (going back to the 'home' screen 'jitters' at the end of the animation as it resets the image for the parallax effect), but I expect that to be resolved in updates. Some inconsistencies with the keyboards too, but devs will have to update their apps to use the new 'flat' style.
What I would like, however, is to be able to swipe up and instead of pulling up Control Center, I want the multi-tasker. Ideally, let me pick what each directional swipe can do... Maybe I want to double-tap home for Notifications, pull down for Control Center, and swipe up for multi-tasking... 4-finger swipe up on an iPad is okay but I find my fingers 'skipping' on the way up a bunch if my hands are too dry or something, causing the gesture to not be recognized.