The idea is fairly cool...however, the ad banners at the top are incredibly lame. Additionally, it messes up the text color in shops, lookups, etc., which is VERY annoying and troublesome. People with dark shop backgrounds could get in trouble!

Also, each one seems to alter layouts differently--I just fixed my guild's layout after they revamped the yellow sidebar and it's totally changed (the div tags and whatnot) with the halloween one.