I was a dyed-in-the-wool Android fan and user for a long time, mainly Samsung Galaxy devices. I loved being able to make my device my device, instead of being like everyone else's. I host my domain with Google, so the services integration was fantastic, Google's data mining practices notwithstanding.
Then Handoff/Continuity came out in iOS 8. I tried an iPhone 5s and was blown away at how well it integrated with my MacBook Pro and made my life soooo much easier, both at home and at work. Now I wasn't tethered to my phone as much. I could read/reply/manage my text messages right from my laptop, could make and receive calls on the 'puter, switch to the phone if I needed to head somewhere, and all of the Google Services worked as well on the iPhone as they did on the Androids. Not too mention how well it integrates with my Apple TVs. Apple Pay and TouchID have just made a good experience better.
I'll just say this now...the iOS UI sucks. I can change the wallpaper and the ringtones. That's it. No desktop widgets, so I have to unlock my phone and open an app to get at any data. The calendar integration with my Office365 work account is horrible, so I have to use Outlook on my MacBook to manage my calendar. Despite these shortcomings, the quality of integration between my all of my Apple devices far outweighs the drawbacks. And now the Apple Watch Series 3 is out, so I would be even less dependent on my actual phone device. This is a HUGE benefit when I am pretty much doing anything outside (running, working on the car, yard work, etc.).
TL;DR - The iPhone integrates so well with the other Apple products I already own (MacBook, Apple TVs) and am currently shopping (Apple Watch and HomeKit home automation devices) that I can't move back to the "just a phone" model, even though I prefer the Android UI.