My wife and I have shared an iPhoto library for years between our two accounts on a single mac. We can't both have the library open at the same time, but otherwise, it has worked like a charm. I was a bit worried about introducing PhotoStream into the mix, and my worries have come to fruition. It almost works, except that every time we switch accounts and start iPhoto it says that the library needs to be upgraded and then it goes through a long process of repairing the library before opening. Once it is open, everything works fine and our PhotoStreams are even mingled into a single PhotoStream import event, but the constant repairing is annoying. Anyone else experiencing this or have any suggestions?