It looks like they did as good a job of splitting the baby as one could, upholding the Proposition but also the marriages performed prior to its passage.
The decision is as sound as one could hope for from the Court: They recognized that while they can get away with a lot in interpreting the Constitution, when the voters explicitly change the Constitution in order to remove any margin for error in interpretation, that's that.
Gay rights activists, predictably, are mad as hell. But they're wrath is wrongly channeled at the Court, which did the best it could in upholding existing marriages. Likewise, they miss the mark if they demonize everyone who voted for Prop 8. The problem, rather, is that the activists ran a lousy campaign against Prop 8. If the gay rights activists really want gay marriage because gays are normal people just like you and me with the same desires as you and me, they need to dump the abstractions designed to hold together a GLBT community whose members range from mild-mannered gays and lesbians about whom no one gives a damn all the way to the circus on Castro Street (in San Francisco) in which gays demand respect from everyone else and show none to anybody who looks even slightly askance at event the strangest of their antics. This is not a matter of right and wrong, mind you. It is simply the politics of building a coalition centered around winning elections on behalf of mainstream gays and lesbians, in lieu of the current coalition designed to maximize donations for professional activists.
So, as an opponent of Prop 8, but also of judicial activism, I offer a regretful tip of the hat to the Court. They came far closer to a responsible decision than I would have expected. I hope that gay marriage supporters will now show some responsibility of their own, dropping the hysterics that turn away prospective supporters, admitting to the lousy job they did in the last campaign (winnable, were it better undertaken) and taking seriously the need for a better approach in crafting a new initiative and selling it to those who are squeamish about but not inimical to the idea of gay marriage.