Forums only work well when they are moderated heavily and there is an outstanding search feature. I cannot stand getting into forums that have the exact same question replicated over and over and can't filter them out.

Any system also needs to take into account deprecation of information. That's always something that takes a lot of time and effort, but is hugely important.

I like the idea of using a forum or a ticket system with publicly viewable tickets that the software owners then use to feed a comprehensive FAQ. This allows for the benefits of a FAQ, but also allows you to see current issues/resolutions that haven't yet been absorbed into it.

Forums also work well if you can benefit from crowdsourcing the support :)