I have the 2011 version of the T-Mobile HTC HD7 which is the AT&T HTC HD7S (the "S" stands for S-LCD screen it uses) and I'm running a hacked version of Windows Phone 7.5.8107.79 whereas AT&T just simply refuses to update the handsets on a timely manner. In fact, AT&T won't even offer Internet Sharing via default when T-Mobile enabled it which is why I flashed a T-Mobile ROM over the AT&T ROM to get the latest firmware/software updates along with Internet Sharing.
Now regarding the Nokia, I am in love with it, I am on Month 8 of my 2 year contract and have been trying to figure out ways to obtain the Nokia on a low price and extend my contract (I'm still a noob in the handheld-world, if anyone can help me out that would be great).
The operating system is stupidly easy to use and configure, I have a iPad 2 so I'm familiar with both OSes, I honestly find Windows Phone 7.5 MUCH more enjoyable to use and play around with than iOS 5.1. It is SUPER stable and secure, literally have like 50 ongoing Live Tiles divided into categories and with all that information being processed, the phone has never frozen or locked up do to the constant scrolling and opening of the Live Tiles processing onscreen information.
Facebook, Twitter, and Windows Live are integrated into the People Hub which is the Tile with multiple icons in it on the screen you provided, it is fast and smooth with quick access to everything. If you pin the Me Tile (which is your own little account where everything is contained) you'll get Facebook-like notifications on the Tile and you can access your status updates etc.
I can provide more information if you want, just ask.