I think too light, but I don't think I've ever got the concept straight in my head. I thought it was a mapper from a URL parameter to a function - the first two code examples in http://web.archive.org/web/20130402141837/http://edorian.pos... are what I understood it to be.

TL;DR: I think Silex has changed a lot since I looked at it. I can't give you a good answer.

Silex is definitely a good lightweight alternative. For enterprise work Symfony 2 is a better choice, but if you need to throw things together (but need to keep the option of adding more features without writing them yourself) you can easily integrate a bunch of the Symfony 2 components into Silex.