I've had every iPhone since 3GS.  Tried the Galaxy S3 when it was the hottest thing.  Back to iPhone I went in 2 weeks.  Then tried the HTC One (M7) when it was the hottest thing.  That one went a bit longer, a whole month before I switch back to iPhone.  In both cases it was pretty much the same issues that caused me to go back - battery life, inconsistent UI smoothness (as in lag), random app crashes (most frequently Google Play store app).
Fast forward to March this year.  Gave the LG G2 a try since it was free.  I couldn't be more pleased.  Smooth as butter, no more app crashes, and battery life the iPhone can only dream of.  I held onto my iPhone 5S, sat idle for 2 months before I sold it.  I actually went and bought another Android phone shortly after I sold the 5s, the Sony Xperia Z1 Compact.  Same G2 smoothness and even better battery life, in a smaller package with build quality and material near Apple's standards.  The Z1C is now my daily driver as I prefer the smaller frame.
I am intrigued by the 4.7" iPhone 6, and chances are I will burn my upgrade on it (I have 3 phones to do cross upgrades from 

), but if I decide to switch back, it'd be as hard as the switch from iOS to Android since I've been spoiled by the niceties on Android.
The one thing that I do miss from iOS is iTunes, for its brainless one-click restore when switching phones, which is not possible on Android, and the ability to store the apps I bought locally on the PC.  It is a painful process to switch phones on Android, having to redo every single setting and redownload every app, and more often than not lose the data in the app.  I know it's possible on Android with apps such as Titanium backup, but it's far from simple one-click as on iTunes and rooting my phone is something I'll never do.