The point of OOP, IMO, is NOT forcing anyone using the code to have to go in and modify a method or class. Good OOP allows you to easily hook or extend a new functionality w/o touching the core class. Back on subject, I have been using PHP5 since the first beta and it is by far better than 4.x. With a few exceptions, I can run most PHP4 code within my PHP5 environ.
Thats why I'm against going all OO because the majority of PHP programmers can't modify OO code because they don't know how to.