The great thing about open source is that there are usually a ton of open source projects to help you solve a problem. The bad thing about open source is that there are usually a ton of open source projects to help you solve a problem. Part of a good open source developer is the mentality to test and evaluate different projects and then choose the best option for your current problem.
This goes along with my research phase. Projects with good documentation are much easier to work with. If the project leaders have not written a lot of documentation, then they will most likely not be very forthcoming with support requests. Sometimes programmers put the documentation inside the code itself and solely rely on things like javadocs. This is nice that they wrote something, but digging through javadocs can be a pain because it is often hard to piece together how the classes interact.
The project community is another great place to get support. I look for projects that have active communities with participants outside the sponsoring company. If the community is just full of employees from the sponsoring company, I worry that the project has not been picked up by a lot of people. But a vibrant community is a great thing because people outside the company start to take ownership of the project.
Project Lifetime & Release Cycle
Look at how long the project has been around and how often it releases code. A project that has been around a few years with regular releases is probably solid. A project that hasn’t released in five years is probably dead. I don’t mind using really new projects, you just have to be careful with them. For new projects, I put a more emphasis on the project momentum.
Project momentum is probably the most important factor for me. I want to use an open source project that will be active throughout the life cycle of my development project. Otherwise you start to have the situation of supporting legacy code that no-one is familiar with. Momentum is tricky and you have to take into account the overall open source atmosphere. What trends is the industry following? What types of technologies are now hot?
A good example of this is Struts. Struts became the standard for java MVC programming a long time ago. There are a ton of projects and programmers that use Struts. But Struts is now on the way out. People have figured out ways to design frameworks that extend the functionality of struts. Would I choose Struts for a new project? No. I feel that a safer bet would be to use a newer, more up and coming framework like Spring MVC.
So evaluting your open source options is not a fixed science and involves some fuzzy math. In the end, choose a library that you are comfortable with and you think will have a longevity in the industry.