"I am SO not happy with the ios9 upgrade. I too have a 4s and it's been a nightmare since the update. Slow, battery not lasting - pebble (original) watches no longer work with iPhones."
I understand that you must feel frustrated. By agreeing to the term of condition for the firmware update, you cannot claim against Apple.
You simply must wait for the upcoming updates that will stabilize the software and eventually make your Pebble compatible once again.

I didn't do the update straight away, I did it a couple of weeks ago so missed the opportunity to downgrade.
I wasn't aware that Apple offered a possibility to downgrade.
Yesterday and today the phone freezes within a minute of trying to use it - have to soft reset. Black screen - have to soft reset.
Have you tried restoring your device and then set it up as NEW iPhone?
Then you can recuperate all your data manually (without restoring through a backup).
So your girlfriend is NOT alone in wanting to downgrade and there would be thousands of users experiencing problems.
Granted that many users are experiencing the similar issue. On the other hand, some don't experience such issue.
I am now terrified the phone will completely die on me. I have been a huge apple fan but seriously am considering an android as I resent having to fork out another $800 for a phone just because Apple don't give a **** about their customers who don't have the money to buy a new iPhone on each new release..
Again, it is a choice you're making. Apple orchestrated plenty of solution for us consumers to get the new generation of iPhone.
For example, that new program of financing you can undertake which was spoken during the keynote.
Apple Retail store also offers a recycling program where they buy your old phone and they provide you a gift card for you to buy a new phone.
If Apple didn't really give a **** about us, they wouldn't offer such solutions. I understand that being livid clouds your judgement and your rationality. Remember, Apple is here because of US consumers. I really doubt they would do on purpose to anger us.
NOT HAPPY. I sympathise with you and your girlfriend.
=(