As someone familiar with the Microsoft stack (C#, .NET, VS, etc), I found the going pretty straight forward. If you're familiar with the Microsoft development stack, you should probably have a fairly easy time getting used to the platform. If you're familiar with Silverlight (which I wasn't), you should find it very easy indeed.
I had a bit of a minor hit with an app I wrote (see http://www.techflash.com/seattle/2010/11/windows-phone-micro...). For a couple of weeks, that app was #1 in the paid tools category and #35 across all paid apps.
As someone familiar with the Microsoft stack (C#, .NET, VS, etc), I found the going pretty straight forward. If you're familiar with the Microsoft development stack, you should probably have a fairly easy time getting used to the platform. If you're familiar with Silverlight (which I wasn't), you should find it very easy indeed.