I like the idea behind the circles. By sharing things to some circles I wouldn't spam people with uninteresting stuff. For instance, if I setup a Gaming Lives circle then I could exclude them from when I share GL tweets, etc. as they already see them multiple times currently. Likewise my cool, hip 90yr old gran that somehow magically loves all things social media can be excluded from any sharing of content that may be offensive to her generation.
If this takes off then I'd probably use it over fb/twitter, in fact I'd probably kill my fb account altogether.