I'll go ahead and give a quick review of my experience with iOS9 on my 4S that I've had since launch.
Honestly I love it. I went ahead and ordered a 128GB 6S on Saturday because I honestly couldn't stand it anymore. But out of curiosity I did a clean install of iOS 9 and it performed beyond my expectations. I'd say if you're comparing a clean install of 8.4 and 9.0, it may be everrrrr so slightly slower, but not enough to matter. 8.0 was a complete disaster and I had to downgrade that on the same day.
Anyway, I love the new wallpapers, and the News app is amazing, so much that I moved it to the bottom row in place of Safari. I had completely give up on surfing the internet in iOS 8 due to being too slow and pages reloading, but the News app is so fast that I can actually use my phone to read web pages again. I love the new Search functions, that was done really well. I love the new way you're able to go back to the last app you were using. I use the Notes app extensively so I welcome new functionality there. Being able to search your settings is pretty great, as well as seeing what is using the battery the most.
The only thing that is really pissing me off right now is that transit directions aren't live in iOS for NYC, and that was one of their main selling points. But iOS 9 actually runs really really well on the 4S considering it's advanced age, and I honestly if I new it was going to run this well, I may have held out a little longer on getting a new phone. I will say that the phone is 64GB and is pretty clean right now (all I've done is download Spotify and Runkeeper), so your mileage may vary. But for me right now it's a surprising pleasurable experience, and I could definitely see myself running iOS 9 on the phone another year. My main issue honeslty is that my phone is old and has multiple things wrong with it, so I don't see the hardware itself lasting that much longer. But it's had a great run, and I'll probably keep using it for jogging until it dies completely.