Autosave works fine for me.
I also use versioning set to four copies and I'd recommend you to do this as well.
If feels safer to know that when Sonar saves, it doesn't save over the actual project file, but instead creates a copy of it.
 
If Sonar then crashes when auto-saving, your project file won't get corrupt. The latest auto-save-copy will and you'll have another earlier copy to go back to.