I have written my own cms (EdMin Webmanager) because most of the cms systems I had seen were to heavy/complicated etc. for clients to work with. I made a system what is fast and easy to use; that’s what is important for most people.
I used to use e107 (and still do for some projects), but had actually been using PHPBB recently to handle user registrations and data as well as my forums and building my database driven content around that. Since the upgrade to PHPBB3, they changed the way users and sessions are logged in, so I started building my own. I got the user data and all that set up right, complete with privileges, but got stuck designing the forums properly so I put that project aside. My question for you guys is, do you recommend using a CMS, or should I stick with designing my own?