I got a WP7 phone (the HTC HD7) after realizing that I had a top-of-the-line Android phone (the HTC Evo) and, even with CyanogenMod, it still felt really slow. Almost worse than an iPhone 3G. For me, WP7 feels a lot better than Android or iOS.
Even having skipped an entire sprint (~10 days) to focus more on Android, the WP7 app is progressing along further and faster than the other clients. Just having support for (albeit limited) databinding is a huge win. Plus, you get great looking apps pretty much for free with the theming defaults.
That said, I have access to a few Windows phones (HTC Surround, HTC HD7, and some keyboard phone) and I would not be able to switch to any of them over my Nexus One. Dealbreakers for me:
* The search button is locked to Bing (not configurable)
* The browser is not only IE, but a crappy offshoot of an old version of IE (I believe IE7 w/ a few patches)
* No Pandora
Also, since you are attacking simultaneously on four fronts - a blog post about the advantages and disadvantages of developing code and assets (graphics) for each platform would be great (obvious one - no IB/ExprStudio for Android?).
As for usage stats, I'd have to check with the client (not sure they'd want that public).
Also, I may be completely overstating how slow it was. It definitely wasn't unusable, but it felt slower than it should have been. This was after multiple wipes to multiple ROMs too.