You won't be able to install a new 10.9 guest - upgrades may or may not work.
VMware doesn't support Fusion 5 on 10.9, but it generally works. You'll miss out on things like the new power management options, full screen capabilities and such. For occasional light use, it's probably ok, but if you're a heavy user, it's worth the upgrade price (or if you do a lot of 10.9 guest work).
