My 925 just arrived from Newegg this afternoon, as did the red wireless charging cover I ordered from Amazon. I'm currently running what I hope is the final and most recent update of Black.
I can't speak on Wi-Fi calling because this is my first T-Mobile phone to have it. My last phone was a Rogers 920 which I still love (I actually think I prefer its screen to the 925's). Might switch my SIM between the two phones. We'll see.
I don't think I'm going to attempt to use a case with the charging cover. I had a ***** of a time removing the cover earlier. Ouch! I thought I was going to break it even though I removed it correctly.
Oh, and I have the Simple Choice plan with 500MB of data. I share a plan with my mother and our bill is $90 and change each month. Recently, my best friend said something about wanting to join in on my plan. That would bring us to having to pay even less between the three of us, which would be GREAT.