Badly designed, full of obsolete information, lacks recent information and documentation for most WordPress functions... I've found that the easiest way to find info on how wordpress works is to read trac tickets.
It's quite amazing to have a documentation that is so bad when at the same time you're using PHP, whose documentation has been a reference for about ten years now.
While in many ways the Codex really sucks, I've found that the longer I use it, the better I become at guessing where things are located. I primarily use it for the function reference, which can be searched pretty easily with Google.
That said, there are times when I get fed up with the Codex and just dig around in the source code instead.
Where is the "its badly designed and hard to navigate" option?