I made the not-so-tough decision to downgrade to Version 5.0.3 today. It wasn't difficult, because I had left myself properly set up.
You will need to have an installer for the latest version of Fusion 5. Again, 5.0.3. I keep old installers around, so this wasn't difficult
I also had my license code for 5 saved in a text file but it turns out it wasn't requested.
What I did, and I suggest anyone attempting this, to follow these in order. DISCLAIMER: I do NOT claim this to be endorsed by anyone else, and that you run any risk. You should copy your VM to another place before proceeding.
• Read everything before doing anything. Look through all of the steps below to make sure you understand them before proceeding.
• Shut down (do not pause) your VM. After that, copy it to an external drive for backup.
• With Fusion 6 still running, select your VM, then go in to Settings. Click the "Compatibility" pane.
• VMWare 6 uses Hardware version 10; change it to version 9 which is the top setting for Fusion 5. Click "Apply".
• After that process finishes, quit VMWare 6. Locate your VMWare 5 installer .dmg, and your license code for that.
• Double-click the .dmg file to mount, then double-click the icon in that image's window.
Fusion 5 will install and use the same dock icon that 6 had been using. If you had chosen 'Prevent App Nap' in 6, that may be copied for 5.
• Fusion 5 may launch on its own. If it doesn't ask, just start your VM as you normally would. Windows may require you to re-activate Windows.
Why did I go this trouble? I had just plain gotten too sluggish using Fusion 6. I have a mid-2010 MacBook Pro, which runs Mavericks fine, but I couldn't get things to process quickly enough. When I'm in my Windows environment needing to use the software that is only available on that platform, I need some reliability and promptness. Perhaps a newer laptop would run Fusion 6 at a more bearable pace, but when Apple only uses Retina displays as a precondition to using the new Haswell processor I'd been holding out for, it got priced out of my range, for now.