I am with the zealots, on the whole. Already thinking how to work this tech into my projects.
One concern is that the federation model is not yet stable. From the draft protocol spec :
"2.5. Wave ownership and server authority
The operational transform used for concurrency control in wave is [currently] based on mutation of a shared object owned by a central master. "
Reading here and elsewhere in Wave's docs, it appears that (elements of) Google would like Wave to be more federated than it currently is. Because this uncertainty lies at the root of the system and because 'architecture is politics' , we might expect a lengthy period of wrangling before a standard becomes stable.
Either way, I hope the notion that Google must sit astride all Wave data has been put to bed. That is simply incorrect. Wave is truly an open system in principle. In practice, system designers and application owners will need to step up and compete with Google if it is not to remain a de facto Google platform.