A must have!  The way they have done expansions for Eldritch Horror just blows Arkham Horror away!  Now the game tells you when you want to use a sideboard or not (or you can choose the adventure or opponent that uses it) so you are not overwhelmed with expansions.  Each expansion shows up to give your games a unique flavor.  We just love the new preludes, and while it seems that they give you an added advantage, in our games at least, that is only an illusion - our win loss ratio has not changed.  In fact,  we beleive the extra "boost" that sometimes the prelude cards give is more of a "false hope" as the doom we are prearing to face is even worse then we orignally believed.