Does it do what YOU need it to?
How fast is the code?
How large (file size) is the framework?
Is there adequate documentation?
How active is the community?
How modular is the code?
Frameworks can often get quite large in file size and its likely that you don't need all of the code. A good framework will keep the code modular so that you can pick which functionalities you need.
Which professional websites use the framework?
A few of the frameworks are being used by some recognizable websites and business. For example, Digg and Apple use Prototype/Scipt.aclo.us, cNet and GameSpot use MooTools, and WordPress uses jQuery. Big company support can show the maturity of a framework.
How much time does it save you (how short can the code be)?