I've learnt that Joomla is a b*tch to upgrade from 1.0 to 1.5.
They've discontinued 1.0 from the looks of it, which leaves most 1.0 users in a hole in terms of security.
As for building new modules, it's pretty straight forward and there's a lot of resources available to help you build.
In comparison to Drupal, it's very easy to work with. I didn't get along with Drupal which was the main reason I switched to Joomla. In an ideal world though, you should build your own from scratch. Open source is horrible because bugs are easily found and exploited, and if it's not kept on top of (like Joomla) you'll find your website hacked very quickly and easily and constantly. I'd recommend going with Joomla if you want an easier time, and there is a lot of extensions and support, since it's basically the same as Mambo.