One problem I have with open-source is not often the product or documentation, but the level of community involvement and support. Response time for some questions on official forums can take more than a week, which can honestly make a big difference when choosing a platform for commercial services use. However, I find that evaluating the community beforehand is very difficult. How do I know what are the most common issues, and whether or not people are getting support for them? How do I know what problems I personally might encounter based upon my project, which would require some level of community support? Do you have a formula for performing this evaluation?
If your question is posed because you don't want to pay for support, then I think your best bet is to ask questions about a specific project on neutral forums like this and see what others have experienced in terms of support and timeliness of resolution to issues.
If you are willing to pay for support, there are companies that provide commercial support for open source projects, like OpenLogic as an example that could factor into your formula.
I've usually found that relying on community support for anything mission critical is asking for trouble. So I usually stay away from open source unless it's something that I'm confident we can support in-house or there is a commercial support service available for it.
The only real way to evaluate the community is to try it, participate in the community. Most communities will have good support networks available in one manner or another. Also many individuals are starting to offer support services for the various frameworks as well, for those looking for a more dedicated resource for problem resolution.
But I find the best way is to just get in there, ask a few questions, poke around at the resources, and really see what is there.