I've been using Ning for a few sites. It's come a long way from even a few months ago...
Here are a few sites that have been custom developed away from the standard Ning templates:
The People's Lounge
Straight No Chaser
Ryan and Sean
Z Spot Now
The Twilight Saga
Recruiter Earth (this is my favorite one; both look, categories...etc.)
Many of these sites have thousands of members, some are selling sponsorship/ad space and/or are selling "groups" where content is featured. Ning has many features to easily customize the site. However, to do the really cool stuff, you need to know "CSS for Ning"; they sponsor a decent developers forum, but looks to mostly feature the basics...
Anyone using Ning beyond friends/family networks?