This is my "new" HTC Radar 4G w/ Windows Phone 7.5, I got it around March-ish. Coming from an iPhone, I 100% love this Windows Phone even more. I know it might sound shocking, but I'll give you 3 reasons:
1) Live Tiles and the over all design push the envelope, and bring together beauty and functionality. Not only do the live tiles look good, they also perform as well as an iPhone app.
2) Facebook integration straight into the OS! Although Apple recently took on Twitter in the 5.0 update, having Facebook updates and contacts directly in your "people" tile is amazing, not to mention that Facebook chat is included into your text message feed, so chats and texts from each person are in one single place.
3) Xbox Live! Having access to your Live profile on a phone is no "new" thing, Apple and Android both have apps for it, but only recently has an "official" one come out. Windows Phone has it built in to a Live Tile which includes Avatar customization, messaging, and... GAMES! You can rank up your gamer score straight from your phone with Xbox Live games.
Things I don't like: Well, the only down side is the fact that there is a lack of Apps, I mean, there are 50,000 apps, but compared to the 1,000,000 Apple has (I think), it is dwarfed. And it is frustrating to not have the app that you want.
All and all, I love my phone!