One of the crucial ways in which it differs from facebook is that it doesn't mash all your contacts together. Instead it creates circles, into which you divide your various acquaintances. That way you can complain about work without co-workers seeing it, send family vacation pictures to just family, etc.
It seems quite similar to facebook, though, and this salon.com feature gives a roundup of the assessments of a bunch of tech pundits.
Soon you may be invited to try it out!