It’s good for developers because it makes it easier for them to focus on making their web apps better; they get lots of distribution with a lot less work. It’s good for websites, because they can tap into the creativity of the largest possible developer community (and no longer have to compete with one another for developer attention). And finally, it’s good for users, because they get more applications in more places.
Google’s own social network Orkut has opened up a sandbox where developers can start building applications using the OpenSocial APIs and Plaxo has already implemented OpenSocial on their Plaxo Pulse service.
For ongoing feedback and commentary, check out the ginormous OpenSocial thread that has been developing on Techeme.