I have one, and the software isn't very stable, but the device itself is great, and the display is amazingly beautiful and crisp (640x480, 280DPI FTW). Some things like waking up from suspend is hit or miss (just like on desktops/laptops), wifi setup is not yet integrated with any of the software, and there seems to be a lack of coherence on the development front.
However, it's a very promising platform, and the hackability of the device is out of this world. There are already people who have put Debian installs on their SD card to boot from, and use a full XFCE interface to run it more like an internet tablet than a phone.
The device allows developers to create applications in Gtk, Qt, or any other widget toolkit, using any of a wide variety of languages. Users can ssh into their phone over USB or Wifi, and update/upgrade packages, add new package repositories, and do anything you could ever do on any other "normal" Linux system.
While the current software array is lacking, there is a driving effort to create an overarching D-bus framework (freesmartphone.org) that will allow any application in any language/toolkit to access the same system functions, data storage, and more, as a single set of consolidated D-bus APIs.
Overall, I don't consider it stable enough for an everyday phone, so I have a secondary prepaid SIM in it for testing and development until I feel that it can fully replace my real phone. However, I know there are also a lot more people that already use it as their only cellphone, and they are very happy with it. I wouldn't yet recommend the device for anyone who isn't a Linux junky/developer, but hopefully the platform will stabilize soon enough that I can change my mind.
All hail freedom!