Martin Rusev gave his experience building his own framework. His experience sounds unique given that he suggests planning. The first time through it usually evolves out of necessity. Each new piece then becomes part of what you had before, ever growing until you have a full picture and have a standalone project. Something I thought of while reading it is that he did not mention making everything testable. Unless things are written properly it is not easy to create a mock class for testing. (@todo) If you consider this upfront, it goes a long way in making the framework stable and maintainable.
I do plan on taking advantage of what others have learned as much as I can in my framework. I think some of the most interesting frameworks are Solar, QCubed, and recently Yii. I saw someone talking about Prado the other day, and it is good to see they are still moving forward with a new look and renewed interest. Prado was one of the first PHP frameworks I payed attention to. It seems much more mature than many other frameworks. But I only looked at it briefly, so we will see.