Mybb2.0 and yii

[ZiNgA BuRgA]

If one can actually write a good framework, and you don't really have time constraints / don't mind wasted efforts, one would say that writing your own would most likely be better as you can tailor it a bit to the application.

But of course, redoing what's already been done is rather sucky, so unless you need something really specific, it's usually better to just use a framework for a largish system.

Downsides of frameworks would probably be that I find some things are sometimes over-engineered for what you need, which may not be too big of an issue with compiled languages, but something like PHP which has to dynamically grab everything, can have a fair impact on speed.
Sometimes they can take a fair while to learn.
You also may have issues where you want to change something slightly, but feel obliged not to modify the framework's code.  Or perhaps having too much a focus on 'good design' can sometimes strongly impact performance for some things you want to do.