I'm using CakePHP for project at the moment -
www.cakephp.org. I'm not nearly using all of it's features, just basic functionality such as mysql interaction, form creation, form validation ect. and it really speeds things up.
All I can say is that it speeds up your "coding" dramatically. Creating forms, collecting the input, validating the input, displaying error messages, inserting into a database ect. gets very very easy. To see it for your self try this CakePHP tutorial:
http://manual.cakephp.org/appendix/blog_tutorial - it pretty much shows the idea of CakePHP and to some extent MVC framework.
A general benefit of MVC's is that you keep your businesos logic completely separated from your presentational code. This gives you the option to have one "page" which does something but alot of views which presents the "results" in different ways. One view outputs a PFD, the other HTML and the third XML ect...
