I went from BlackBerry (10 year crackberry addict here) to Android and then to iOS. I'm a life long Mac user so the choice of iOS was a natural fit. I actually resisted the iPhone for a long time. My first iPhone was the 4s. Nothing integrates into a Mac environment like the iPhone (naturally!). I was sick of workarounds that broke down constantly, like using Google to get my BlackBerry or Android phone to sync with iCal and Address Book. Plus I was constantly fighting the funky re-formatting it caused.
I was quite the passionate fanboy with BlackBerry. I must have had a dozen models over the years. I've never been passionate about the iPhone like I am with just about every other Apple product, but the iPhone just plain works. Using iCloud to keep my Macs and iPad and iPhone in sync as far as contacts and calendar is wonderful. And it's instantaneous! Having that simple feature, and having it work wirelessly, is addictive.