It is the best CMS out there. Most easly customizable and most flexibly written (unlike robust Mambo/Joomla).
You need a bit more knowledge of PHP and HTML than that what would you need for Joomla, but you don't need hours of digging thru poorly written code to change some simple thing.
I personally like Joomla, mostly cause of the skins theyre good. The best way to know whats best is what YOU think is best. Id suggest trying them all or going to their demos.
My personal choice would be e107. if I were to choose Mambo or Joomla, I would go for Joomla as it's from the same developers behind mambo. As for modding and skinning, e107's simplicity beats most cms out there. Give it a shot