Which of the popular MVNOs have an active user forum? Have folks found them to be helpful?
Generally no. Mainly because there is no need for then. All MVNOs maintain usable online help and can be contacted.
Many of the major national carriers, like AT&T and Verizon, have user forums.
I looked briefly at the Ting forum and it seems helpful.
The thing that is interesting about it is that employees participate. That is really useful since for many things only employees can provide accurate answers.
Probably why Ting is so expensive to pay for all of that support/interaction. Just how much you need it is questionable. Probably worth it if you need support sometime! Otherwise not.