From a bang your head on it until it breaks works perspective, most of the XML works pretty well. The major problems are finding things in the Wiki (e.g., Ricochet!), the few parts that just don't work (e.g., PeriodicMovementMode), and the awkwardness on where and how specific characteristics are made (at the level of the game entity, system, bullet, or bulletpattern), (e.g., the speed of a shot is defined at the system, most of the time). There are some deprecated and missing entries still (Ability Points, anything about floors.xml and particles), but they aren't that big of deal. Getting grep to work on my local install and getting a copy of the wiki helped a ton too.
Its just when I want to do something Ptarthian that things are troublesome. It would probably also help if I managed to get the NonSim parts down.