Consider the following 7 factors when evaluating a third-party library.
1. Efficacy: How well does it work?
2. Performance: How well does it perform?
3. Reliability: Can I depend on it?
4. Ease of Use: How much effort does it require?
5. Flexibility: How many things does it do?
6. Aesthetic Appeal: How aesthetically pleasing is it?
7. Time Sink: How much time do I have to give up to maintain it?
How well is it written?
Can I customize it easily?
How responsive are the developers in fixing bugs?
Does it support the latest version of the language it is written?
I recently had to use Customer instead of User for using a Ruby gem. The assumptions made by this library was not applicable to my project. I sent an email to the author. He never responded. It is better to develop your own library if you can simplify the existing library code.