I hope the idea of circles catches on, and that they are very expendable, I would like to have for instance a Circle called Music where I could add those friends which I share music with and that it would become a central hub which we could all update. I like this idea, rather than the mess that is the Facebook news feed.
Adoption will be slow I reckon as people don't have a huge reason to move this, but thanks to its integration, we will see a number of people use it almost unknowingly.
I like Google's full on approach here, and feels more complete than buzz and even wave.