I plan on buying a device that meets my needs at the time, regardless of the ecosystem. I am entrenched in Windows Phone, Android, and iOS. I am OS agnostic, free to take a similar experience from one device to another. I'm curious to see how well the Lumia 950 / XL will fare against the iPhone 6S/6S Plus, and the myriad of Android devices that will launch in the coming months.
With Apple, you know you are getting one (now two) solid mobile devices released every year. While the spec sheet may be slightly improved every other year, you know the phone you buy now isn't suddenly obsolete after three months. You have to realize that Apple isn't the "we were first" company. They are the "we can do it better, prettier, and charge more money" company. The iPhone models I used may have had their quirks - all phones do - but the user experience was consistent and I was never left wanting more control over my device than what was allowed. I didn't feel the need to change colors and fonts. I didn't want to run a custom ROM.
When I jumped on the Windows Phone bandwagon with the Lumia 900, then the 920, 1020, and so on, I bought into the less is more concept with the design language and more modern approach to a mobile OS. The apps never really came to Windows Phone at the speed I would have preferred, but why would anyone spend more money to develop for yet another mobile OS when Apple and Google provide the most revenue? Over time I began to fall for the flashier looks and higher-end specs of flagship Android devices. Nokia Windows Phones had the better screens and optics, but that doesn't mean much if the apps you want to use aren't yet available, not to mention if they become available. When Microsoft bought Nokia, I was hesitant to think this was a bad idea because of the leverage this would mean on the next round of Windows Phones. I thought this would have been a move to give a more Apple-like experience with a stable OS and flagship devices. It hasn't appeared that way.
I own a Lumia 640 that I use on AT&T. I believe that the size of the phone is correct - manageable from a daily use perspective and easy to whip out and begin using quickly. I found the 640 XL to be too large for my tastes. I just wish Microsoft would have taken the optics on the XL and put them on the 640. That would have made for a more compelling device even if the cost was $150 off-contract. I'm not ashamed to show off my Windows Phone to anyone who asks. I think it's a great device and does just as much / if not more than counterparts that cost hundreds more. Yes it has its quirks. As I stated before, all phones have their quirks. I haven't yet used a device that I didn't hard reset / factory reset at least once. And that doesn't factor in rooting, ROMing, jailbreaking, and installing 'Insider Previews'.