I upgraded my Dell Axim X50v to Windows Mobile 2005 this evening. The installer was really badly written, and it lost connection to my PDA twice, requiring that I hard reset the computer each time. Third time around, I went through and killed every single process and service on the machine, and finally got it to complete the install.
Now I have to reinstall all my software and stuff. Argh.
The nice thing about WM2005 is that it stores everything in Flash ROM instead of RAM like all previous versions back to CE 1.0, so if the battery runs out, you don't lose everything. That alone almost makes it worth all the pain of upgrading.