It's got some nice features, but honestly it kinda sucks as a forum. The layout for threads and subsequent posts is not very easy to read. The big question is, what's more important? The ability to easily browse topics and discussions or the ability to post pics and make friends? I'm not saying this set-up won't work but if you look at the biggest, most well-established forums on the web (at least the motorcycle ones that I'm familiar with i.e. SVRider, gixxer.com, etc) none have this kind of layout. The true test will be whether or not those that were regulars on the old site start using this on a reular basis. If the activity drops below normal you'll know people aren't digging the new site. We'll see.
I agree, the forum is a little tricky to get used to, although I'm sure the Ning developers are actively working to make it better. Making the switch is somewhat motivated by a resource issue. I simply don't have the time to keep phpBB up to date, or to keep the spammers at bay (You might have noticed the old forum had 1400 members, 80% of which were fake). With Ning, I don't really have to worry about that.
I do however think Ning has some cool features that are useful to this type of group. Localized groups with calendars for example.
Lets give it a try for a while; we can always switch back if we don't like it.