I have a 6s and upgraded to 13 yesterday (through iTunes) and its working very well. I was a little nervous because I only had 5 GB of space left on my 6s. I heard that it’s a smoother process using iTunes so I chose that instead of over the air.
I’m actually really impressed with how well it’s been running, it actually feels faster than iOS 12. I really like dark mode and for the first time I can create a me emoji something I couldn’t do before on iOS 12 on my 6s.