Thanks Martijn! Yes there's many high quality modules as also exists in other frameworks or CMS' and I also think it depends a lot on the system and community. Yeah the simplicity and flexibility in PW is astounding and almost cries out "EXTEND ME!". The fact that it is a framework that is so well designed and using that foundation, the CMS as such is built using it makes it very versatile and logik. But the best part is that it is built modular using different types of modules and mini systems (process, fieldtypes, inputfields, js modules ...) and in the exact same way you can extend it and build modules that work exactly the same! You can take almost any module out from the wire/modules/ and move it to the site/modules/ folder modify it to your needs and install it. To go even further it uses same API as you use (or can use) in templates, the transition is almost seamless. So you can also look at core code and modules as reference and example.
I'm not the very advanced php programmer and always got my problems with OOP, but after digging into PW after seeing how modules are done it took me no time to get into it and already built 15 modules in 1 year, AND IT WAS FUN! I'm the guy in love of least resistance and having fun . After using MODX 1-2 years, all I got was a half backed test module that had no purpose but was damn hard to setup, but also didn't attract me as much as PW did. Same goes for other systems that seem like having extra hard layer that's designed for modules, but too complicated to be fun, or like another system inside a system. So even if may not perfect in all regards PW is a great foundation that will show and have impact on other systems. MODX already is announcing a light weight distribution with 3.0, and I'm proud modern systems like PW played a role in it.