To me this sounds interesting. More and more of my computer tasks are managed in online services that I can reach from every computer available. So for me this makes the service more accessible and more useful.
And I almost exclusively use MacOS as my computing platform. That and iOS.
I do realise that I am not representative though, since I pretty much have a couple of computers with internet connection available at all times both at work and at home. And all my friends have computers, so if we're gaming at their place then even if I forgot my portable(s), I can use their equipment.
The one thing I'm miffed about is the Silverlight thing. Makes it not compatible with the iOS devices at my disposal. Ah well, they will move to HTML5 in the end anyway.
As for storing my characters, I would just print them out. No use having a datafile if I don't subscribe to the service. And paper character sheets were ok for me to "store" PCs on before this announcement, and this doesn't change that.
So it's interesting and probably more useful to me than the stand-alone.
/M