Last but not least, the I like it factor is extremely important. The question is not whether you like the library but rather how much the developers like it. Developers will like a library that is easy to use and fun to work with, and this will result in low cost. If the library is hard to use and developers do not like it, then they will not learn to use it to the level of profession required for good quality, only to the level that is just needed. The end result will be suboptimal software.