I have two big networks on Ning (well.. one big and one wanna be big, lol)...
Well, for money paid, I would say pretty good stuff. You can have a very good hand of tools for 20$ / month. You don't think about managing servers, bandwidth etc..
You can fairly easy promote your stuff to your members by sending them messages or sharing content from the site.
Bad sides - horrible looking and functioning forums, not being able to completely export your member (you can export everything but passwords, I know passwords are protected everywhere but when having full control of the site then there could be a way to decrypt passwords)...
There are spammers out there building profile links and sending messages but all links are nofollow so I guess they don't know that, lol
Overall, pretty good platform for money involved