Awesome, can't wait to try it! Circles sounds like just what everyone wants. I think I'd switch to any half-decent social network made by Google. I do hope they provide a way to use the Facebook data export to make switching easy.
I don't know anyone who really likes Facebook; they've alienated users with privacy missteps and stupid interface changes over and over and over. But everyone keeps using it because that's where everyone else is. Somehow you have to break that cycle.
If anyone can do it though, it's probably Google. They effectively broke the back of the fat-and-lazy AOL Instant Messenger (at least in my experience) when they built chat into Gmail. Once they did that, AIM went from a "must-have" on most of my friends' computers to something they barely remember their login information to anymore.
Here's hoping they can find some leverage to get a critical mass of users over from Facebook.
I think that Google can leverage adoption by targeting android users and creating a good application for them.
EDIT: Orkut still is really popular here in Brazil but the majority of my friends migrated to Facebook and do not login in Orkut anymore.
Also, Circles might help mitigate the network effect advantage of Facebook. Even if only one of your real-life circles (say your early adopting tech nerd friends) sign up at first, it can provide some functionality while it is still being more widely adopted by other groups.