In terms of supporting full marriage equality, there's no dodging how late to this party Hillary is—she's doing it after Barack, after Joe and after Bill (in reverse order), and mere days before the long struggle that "isn't over yet" might be effectively over, pending the Supreme Court's decision...
But in the same way it irked me when the anti-Barack Hillraisers seemed to vivisect President Obama over every slight, real (and some were) and imagined (and some were), I would be annoyed if any good Democrats would hold her tardiness against her. Sure, it would've been swell to have her come out for equality long ago, but doing so now is exciting in that it clearly signals her interest in potentially running for president again in 2016—and winning.
Hillary's lateness (political calculation? deference to the Obama Administration's timeline?) does not substantially affect my support for her, and—barring unforeseen shockers (which have been know to befall people named "Clinton"), I believe the presidency is hers to accept or refuse.
Meanwhile, check out how Prop 8 plaintiffs/adorable couple Paul Katami and Jeff Zarillo see their case at this point in time: