I'd use the AGPL if I open-sourced a non-trivial webapp most likely, especially if it were a business rather than a side project, but probably not if it were just libraries or something of that sort.
It makes sense to me as a GPL analog in the webapp/SaaS space. I think of the GPL as informally saying that you can do whatever you want with my code, but once you ship a derivative work, you have to release source to your modifications too. In webapp space, what does it mean to "ship" a derivative app? Seems like "launch a public service" is the way you ship, since nobody is selling CD-ROMs with thick-client apps on them.
In practical terms, a common alternative to AGPL is just to not release at all, because many SaaS providers are worried that releasing their code will make it too easy for someone to set up an improved rival service, beating them using their own code as a starting point. For example, Gitorious is AGPL, but Github is closed-source.