I've been quite a happy user of the Windows Mobile platform for many years. I took the WM train by buying a Pocket PC 2002 PDA, then switched to PPC 2003 and finally ended up with WM 5 and 6.5. Despite the fact that the platform is far from being the ideal mobile OS for a variety of reasons there are still a lot of pros that makes it the best choice for me: (1) From the technical point of view, the platform is very similar to the desktop Windows, which might not seem very important to users, however, it makes it easy for developers to port a lot of high quality apps. (2) It's been backward compatible for almost a decade. That's a big thing! Since I was able to reinstall all my favourite apps again and again whenever bought a new device. (3) Paradoxically Microsoft keeps the platform rather open for booth developers and users, which makes it perfect OS for custom applications and hobby development. So far good work Microsoft!
Now the Windows Phone 7 is coming, which is completely different story. MS decided to break the existing model so WP 7 is no longer backward compatible! Throw your favourite apps away, you will need to buy them again or forget them completely in the case they are not under active development any more! On top of that, you won't be able to download and install applications directly just by visiting developer's website. Instead, you will have to visit Microsoft's marketplace and buy the apps there. How innovative is this, thank you MS for copying Apple's approach. This not being enough, developers are forced to use .NET/C# (instead of native APIs through C/C++), which compromises performance and mainly cut off developers from established 3rd party libraries.
After such a dramatic change as this I'm curious what MS thinks that should motivate me to buy a WP7 instead of an iPhone. I really have no idea at this moment...
FipS