I've been playing for a couple of days with Ning, in its recent reincarnation into a "build your own social network" playground. It's half-baked, to say the least.
The usability is terrible (e.g. you cannot see how many comments posts get from the home page; it took me one hour to understand how to fix the mention "United States" that kept appearing under my name — why is that a default? — until I realized that there are two completely different profile pages that are randomly linked under the same "Edit profile" link, you just have to be lucky to get the right one; if you're invited to a community and go to the Ning home page, you have no easy way to get back to your community home page, clicking on "My Networks" will give you a very unfriendly "You haven't created any networks yet!", etc.), the design is unappealing déjà vu, there are frightening omissions such as a total lack of RSS feeds in the private communities (a real show-stopper for me!), the email notifications don't work... I lost patience in two days. Apparently, they expect you to develop the missing features — that is not totally absurd coming from the original Ning perspective, but it is from the "everybody can build their social network" promise (as in "anybody who knows to develop PHP scripts").
The good things I can say about Ning are: the idea is appealing, especially the level of customization if you're willing to develop on the platform, and they can continue to bake it until it's properly cooked. But until then, I'm certainly not recommending it to the faint of heart.
Did I miss something?