Each level of lucky charm changes what extra loot a container might hold and what the chance of each loot type is. This never exceeds blue items but can provide stacks of over 500g extra per container.
If you google the topic you will find the exact formulae and item lists as dictacted by your loot table file.
Many people consider lvl 5 better than lvl 6, though only few people use this skill to it's maximum potential. See other posts.
Other than that, 3 offensive rating per level at the moment.
