I was really looking forward to Windows Mobile 7 first, then WM 6.5 when it was announced. WM 6.5 turned out to be a big disappointment, and WM 7 just didn't arrive. Since then I've moved to Android - I've got the HTC Desire and I don't regret it.
However, one advantage that Microsoft does have is the tight integration with Exchange & MS Office products, as well as the MS Outlook Mobile version - one of the best mobile email clients. It really is nice to have your Outlook Mobile synced to your PC Outlook (if you don't want to burn 3G data) as well as your actual POP/Exchange email account (if you don't mind burning 3G data), and have the tight integration with Calendar, Contacts, Tasks, Notes, etc. Makes managing your business affairs very easy and productive. Other than that, I don't see any other advantages MS have. Even these advantages are eroding fast, as other vendors start polishing their interfaces with Outlook, Exchange and other MS products.