The boxed set upgrades Karameikos to AD&D 2E, and also includes post-WotI information and developments in the timeline and in the descriptions.

If you are playing in Mystara post-WotI, it might be worthwhile. If you are playing pre-WotI or doing your own version of WotI, it is of questionable worth, save to be complete.

Unfortunately, the boxed set was a rush job. Jeff Grubb had originally written an overall and comprehensive "Mystara Campaign Setting" book, like the Forgotten Realms book, but shortly before printing, the bosses decided they wanted to simply relaunch it as a series of boxed gazetteers. So it was not well edited; mistakes that occured in the original GAZ, and things that were later changed, remained (notably the timeline problems).

Plus, the new map style is inferior to the classic gazetteer style, and the way illustrations are represented as being "in game art" is, well, cheesy at best...