They work very well together. Exporting V5i trees to C4p creates MONSTER-HUGE objects, though. I decide what to do on a case-by-case basis. Sometimes it makes more sense to do it all in C4 with C4 trees and terrains, sometimes importing a V5 tree or two into C4 is a good way to go, and sometimes doing the whole outside scene in V5 is the way to go. Neither program is a "silver bullet" for every scene or animation need. If you have multiple machines, both programs have network rendering capabilities - that saves hours!