I just got an invite email for http://plus.google.com , presumably from one of my friends adding me .
I was pleasantly surprised to find an awesome welcome experience for what looks like Google's most serious play to stay competitive socially.
After playing with only for a few minutes, 2 things jump out:
1. "View as someone else" feature
2. Their invite UI, that animates, and keeps suggesting users from your gmail to invite, is very slick. Before i knew it i had invited 20 of my gmail contacts into my social circle on Google +.
2. Social Circles, and permissions grouping is emphasized from the ground up. This will be extremely useful if the service scales.