I just upgraded to Yosemite from Mavericks and then upgraded Fusion from 5 to 7. I have two guest VMs -- one for WinXP (NIC deliberately disabled), and one for Win7 (with Office 2010). These two guest VMs upgraded smoothly, VM Tools installed without a hitch, and everything just works.
I'm really pleased at how simple and painless the entire process was. With some previous Fusion upgrades, I've felt compelled to do a fresh Windows install and reinstall all of my apps. I'm not even tempted this time.
Well done, Fusion 7 team!
Cheers,
Steven