It takes 15 minutes to boot. Once you see the desktop, you can open Notepad and Paint. Browsing the web is impossible (512MB RAM can't render modern Javascript). However, it is a fantastic proof-of-concept showing that Microsoft's own phone could run desktop code if unlocked.
If you are looking to attempt this modification, the complete repository of files, active guides, and troubleshooting threads are hosted by the community on the XDA Forums Nokia Lumia 520 Hub .
: Boot into TWRP and wipe the System, Data, and Cache partitions.