The most serious problem with previous versions of Windows Mobile isn't the myriad UI issues (which are collectively definitely a close second), it's the stabil

The most serious problem with previous versions of Windows Mobile isn't the myriad UI issues (which are collectively definitely a close second), it's the stability of the core OS. Every last person I know who has owned a WinMo phone (which is admittedly still not very many people, so sample size warning!) has complained about how frequently they have to reboot their phone. This is particularly egregious when the phone crashes as it sits idly in your pocket, causing you to miss incoming calls until you take out your phone and realize what has happened. In order for Windows Phone 7 to succeed where Windows Mobile failed, they need to revamp the internals of the OS just as drastically as they have revamped their UI. It's definitely possible that they have done it; clearly Microsoft cares a lot about this release and clearly some major parts of the underlying OS are changing. But I personally would not bet months of development time on it until someone outside MS gets the chance to use the phones in a non-demo environment.