Well one thing that I've learnt is that the grass isn't necessarily greener on the other side.
My time with the S4 was great, I picked up Android quite easily and loved the possibilities. I could swap out memory, change the look and feel of the OS, had more options, better file management etc. After using iOS since the 3G, Jelly Bean just felt fresh- and in many ways just made an iPhone feel dated.
But I missed the iPhones for general use. You just set it up and go. The stock mail and calendar work perfectly, some of the apps were just better to use- Spotify in particular is one I use a lot. The Android version is pretty, but the iOS version is just better to use. The Financial Times web app lagged like crazy. It's not really Androids fault, it's the laziness of some developers I guess. I also missed little things like how the remote with my headphones could only play/pause a song.
None of it was really a deal breaker though. The S4 only went back because my new carrier sucked balls (EE in the UK- avoid: they are thoroughly ****). So I'm temporarily back with my 4S until they pull their finger out and give me back my money.
Not really sure where to go next though. iPhone 5S w/iOS7 or whatever Motorola come out with? Hmm...I'll see how it goes.