We're watching the take-up closely, and we're far more likely to do a windows app than any other platform. We understand it's frustrating ( and we're not alone in not supporting windows yet ) not having apps on the platform you've been given, but for us it's a fairly major investment.
Right now, the windows phone take-up isn't great ( esp worldwide ), with an August 2014 stat that Android is 84%, iPhone is 11% and windows is around 2.5%.
The site is fully functioning on the Windows phone though ( via the browser ) - but you miss the rich functionality and the push notifications ; app messages.
June 2015 Update:
We have seen that Microsoft has announced that they are investing in easy port tools - potentially allowing us to port the Android or the iOS app to Windows Phones. We think this is a very smart move by MS, and it will allow plenty of developers like us to finally embrace the Windows Phone platform. We'll keep watching with interest when those tools come out - and are mature enough for us to use. Great first step though.
November 2015 Update:
Looks like the android port has been squashed, but the iOS to Windows App appears more promising. We are watching and waiting. If it comes out, we'll be quickly on it.
February 2016 Update:
Very late January, it seems that Microsoft has now released a preview version of the iOS to Windows app bridge. Nice. We'll take a look in detail by mid-Feb, and this should give us some indication whether this is impossible/possible/ how hard and when. Fingers crossed now.