I'm currently using an iPad Air and HTC One M8. Before this, the only other iOS devices I owned were an iPod Touch and iPad 3. Never had an iPhone before, have been using exclusively Android phones(Nexus 5, Note 2, Nexus 4). I have a BlackBerry Z10 as well, but the less said about that, the better.
Have had Android tablets before(Sony Xperia Tablet Z, Nexus 7 2012, Nexus 7 2013) but honestly, Android Tablets are horrible. There isn't even a Facebook app for Android tablets. What they have is just a stretched out smartphone app.
I also own a MacBook Pro Retina. I really enjoy the seamless integration between my iPad and my MacBook.
Books, magazines, web browsing and just everyday general use are also much more enjoyable on the iPad because of the 4:3 aspect ratio. Android tablets are better for watching video though.
I totally agree with TS. Although I have used less iOS devices than Android devices, I have invested much much more into the iOS ecosystem than I have with Android. Why? Because I just find that the user experience that iOS offers to be much more polished and COHESIVE than Android. I agree with the points that TS brought up. iOS just has a much better ecosystem than Android. It just has much more to offer. Not just on the services(iTunes, Appstore etc) side, but also there are just much more accessories available for iOS devices.
The only reason I haven't had an iPhone before is because I just find 3.5 and 4" to be too small. If Apple does release a 4.7" iPhone this year, I will definitely abandon my One M8 for the iPhone. The sweet spot for screen size for me would be 4.7 to 5.2". I like to be able to operate my phone one-handed. I can do that with my One M8, but any bigger and I would run into issues. That's not to say that the One M8 isn't a great phone, it is. I just want to enjoy the seamless integration between iPhone, iPad and Mac. I also find the display on iPhones to be much better calibrated than the display on the One M8. I value colour accuracy and the iPhone's display just looks better to my eyes, even with a lower DPI. I think 1080p on a 5" display is just overkill.
And not forgetting that iOS devices get updates much faster than Android devices and they are supported much longer. The Galaxy Nexus can't be updated to Android KitKat but the 4S, released in the same year, is on the latest version of iOS.
Having switched to a MacBook from a Windows laptop and having my iPad 3 as the first tablet which I really enjoyed using, I really feel that the user experience that Apple offers to be UNPARALLELED. That, and I'm too entrenched into the Apple ecosystem
Some things I enjoy on Android:
Being able to play HiRes FLAC files out of the box
More flexible storage options
Larger screen (For now)