I hear that MacDonald's is a very popular burger chain, too, but that doesn't mean that the food is (how to put this tactfully...) suitable for everyone's tastes. My point being that "popular" doesn't equal "the best choice".
I'm in the process of trying out a bunch of CMSes on a local server (having been roped in to help with someone else's website for the first time :eek:), so here are my thoughts.
Concrete seems to be easy to use (especially considering that I'm only comfortable with writing the HTML/CSS myself: not WYSIWYG), but I don't feel able to get it to produce the output that I want. I like the Drupal-style admin access on any page, but its way of using editable content sections is sort of odd to me. It's all too abstracted. Early days, though.
I've tried Perch, which would suit me (you create editable pages) and would be very easy for the site owner to use. And it works without a database (uses FTP).
But I'm most likely to use either Drupal or Textpattern, depending on how far I get with Drupal (bit complex) and/or whether my friend can use the Textpattern interface. They both have quite a lot of potential, though Drupal seems like it might be more powerful (should that be needed).
Regarding writing your own CMS: why reinvent the wheel?
Hope that this helps you.