In the USA, one of the most installed user bases of SmartPhones has to be on the Windows Mobile platform, though rarely do we hear much about them. All the HTC and HP devices (with the exception of Danger's Sidekick) are running WM 6.0.
While I own one for my Verizon account, I hardly use it (though Verizon does have the best cell coverage around in the USA it seems) because I'm more brand loyal to Nokia and use their handsets and Smartphones more, along with the Blackberry and iPhone as well. Call me GSM centric.
This gives me something to install when I get home and see how good it is. In the past I've made calls on my Windows Mobile phones and had fairly good experiences over Skype, including one call a few years back to pal Alec Saunders as a test.