SaaS is outside of Apple's expertise so it'll be interesting to see how this goes. I've tested the beta and it's very impressive, but where Apple has trouble is scaling its internet services.
It's amazing how much Google has upended not only the market for, but our expectations of "Office" suite products. When I first clicked the "share" button in Pages, I was actually expecting an "invite a collaborator" option and was disappointed until I stopped to remember what a massive infrastructural achievement Google's real-time document collaboration system is.
I'm sure such functionality will eventually come to iWork, and it's undoubtedly very impressive already, but it's a reminder of just how wide Google's lead is at present.
This service doesn't really seem like it needs anything in particular to scale; it's just a static asset-load of Javascript. All the "magic" is done by talking to the same iCloud backend that the installable version of iWork does, which was already scaled to the point where this would just be a blip.
I'm sure such functionality will eventually come to iWork, and it's undoubtedly very impressive already, but it's a reminder of just how wide Google's lead is at present.