Though, it is sad that they completely adapted a form of Android UX in their own apps. Whenever I open the store app, I feel pain seeing how similar ( and uglier compared to 8.1) it is to playstore. Not to mention those circular loading animations and hamburger menus scattered everywhere :/
If I remember correctly, the circular loading animations have been with Microsoft since Windows Mobile 2003, and has been improved through Windows Mobile 6.5.3... which puts Microsoft ahead of Android by at least three years. Even the hamburger menus are reminiscent of the old mobile start page in Windows Mobile. In fact, Windows Mobile had a huge market share peaking at 2007 - which was the year when Android and iOS were announced.
I can't comment in-detail on the UX/UI of Android and iOS because I haven't even accumulated 30 minutes in using an Android and an iOS device combined, even if 95% of those around me are using them. I also haven't tried the technical preview. I definitely prefer the UX/UI of Windows Phone 8.1 over its competitors - the live tiles, customization, etc. feel natural to me.
A quick survey of those around me had mixed reactions about Windows Phones - a lot were impressed (of course, I'm showing the L1020 and the camera), some didn't know what it was and there were some who were adamantly not in favor of it - the didn't like the interface, it was "confusing", they couldn't navigate, etc... In fact, my first Windows Phone was given to me for free because the original user (and those around her, including some tech guys) didn't understand how to use it, while it took me just a few minutes to figure everything out.
At the end of the day, it's a matter of preference. Being able to add customization options may be nice, but it should follow a standardized and brand-recognizable theme. It also should be implemented natively from the OS, not through some extra shell. It's always risky to choose between lowering* a company's standards to the expectations of the customers and setting a new standard which the customers will look for.
- I chose "lowering" in analogy with the Bell-curve distribution for population-related criteria.