I use Philly Car Share, which was once a nonprofit but was purchased by Enterprise, not Zipcar, though we have Zipcar here in the urban hellhole too. Enterprise hasn't really changed anything significant about how PCS is run, at least from a user perspective. There hasn't really been any perceptible integration with the rental company at all.
I guess the big potential benefit for a rental car company buying a carshare company, aside from various economies of scale generally, is that rental car demand is actually fairly low on weekends but carshare demand is high.
