Interesting, have you used it? The comment thread is not super encouraging.
Unfortunately, I have not. But it's a case of: even if it's not perfect, we might very well have the brain power necessary here to improve it to meet our needs.
I asked Boone, the developer of that plugin, about moving this site. He said, "Good Luck".
I think the size is a problem.
Yep, data migrations like this are very hairy. Sometimes is is better to be selective about what comes across and then write some code to extract what is needed. I would think in the order of priority that user info would be first. That said, this is the kind of thing that needs a spec.
There's a plugin from wpmudev.org for migrating the user data from Ning to BuddyPress: http://premium.wpmudev.org/project/ning-to-buddypress-user-importer
Our university has a membership to this site, as I'm sure others do too.
I'd also offer that data migration challenges are specifically why stuff doesn't move from platform to platform much.
Unless there is sever pain, it should be carefully considered, since moving also inflicts pain.
I'm all for a wiki knowledge base, and am actually a fan of Ning.
I'm a perennial lurker, and have found this an invaluable resource but have to admit I'm guilty of not giving back to the community and not engaging on a regular basis.