So I was actually on 5.1 on a iphone 4s and held off updating until 6.1 came out. Let's just say I wish would have never updated in the first place. I did not have any issues under 5.1 and as soon as I updated to 6.1 I have had bluetooth connectivity issues, wifi dropping, and battery drain. I've done a DFU restore and did not restore from backup so the phone was bascially setup as new and am still having issues.
That being said this will probablly being my last iPhone, aside from what the sheep would like to believe, Apple has horrbile customer support. I've been to there store on few occasions for other un-related issues and they never know how to fix anything. They're standard protocol seems to be for you to do a restore for almost all issues.
On top of the support issues I've grown tired of Apple's incremental updates which have delivered nothing really innovative since the iphone 4. Let's take the iPhone 5 for example, they streched the screen 1/2 an inch, slightly improved the camera & processor, change the connector (so you would have to shell out more dollars for cables and/or new accessories), and finally added LTE. Is this what you consider innovation? I must have my definition of the word mistaken for something else.