I have limited experience with this but I'd say that if collaboration across networks is something you really need, then it's not a bad solution.
Alternatively, there's Cacoo, but that does require a new account setup and as the Morten Just article correctly points out, a major advantage of using Google is that it's fairly ubiquitous.
That said, while it's possible to create fine lookin' wireframes with the Google drawing tool, it hasn't been designed with that in mind - it doesn't feature interactions or masters/pages for example - so it's no replacement for a prototyping tool.
One feature of Google Docs that distinguishes this from the alternatives is that documents can be submitted to a template library, so there's the potential to build up a community-driven repository of wireframes. Would that be a good thing? That's a whole other question.
For collaboration, great.
For everything else, not so great... yet.