God I hate to hear all this whinging whenever a new version of software comes out and someone has to pay for it.
... V9 of Parallels is working fine for me with Yosemite, and I did chose to buy version 10, because I like to stay up to date and it works fine as well.
I generally don't mind paying for upgrades, and often pay my share and more for shareware and donationware that I find of use.
But with Parallels' "support" and slow bug fixing, you're not getting your money's worth: I purchased a full copy of v9 this year (I've owned all the odd numbered versions; no need buying EVERY year), but a bug that remained unfixed for three months after the introduction of OS X 10.9.3 -- a bug in Parallels itself -- left me high and dry for months.
Parallels response: Sign up for our mailing list, and we'll let you know when it's fixed.
My response was to purchase VMWare Fusion and cut losses.