You can buy it now. As stated above, Fences, WindowFX, Start8, Tiles work with Win8 now. If you installed a component that doesn't completely work with Windows 8, you can be reassured that as soon as the update is created to make that component 100% Win8 compatible, you will get it for free if you've already purchased an Object Desktop subscription.
When you buy the Object Desktop package, you get a lot of component programs that would cost more if you bought them separately. Alternatively, if you only want to buy the ones that are currently 100% Win8 compatible, you can do that, too. That's one of the things I love about Stardock - versatility.
Personally, I've been a Stardock/Wincustomize client since 2005 and I have Object Desktop and ObjectDock Plus v2, and I love them.
There is no need to wait, if you don't want to. It's totally up to you.