I'm having problems mainly with video files. I have two applications that handle video files (MPC-HC and JRiver Media Center. When both have registered the file extension, it seems to work ok. When one of them (or both, haven't tried) unregister an extension, DOpus will not respect the "always Open With..." Windows setting (tried .mov, .mkv) and open Sync Center instead (Windows Explorer opens them normally).
It's a known issue on Windows 8. We're looking into it.