This one is a very good product, which is a) an interesting campaign setting (with a twist), b) a fine sandbox c) an campaign creation kit. Do yourself a favour and buy it together with 'An echo resounding' as it is a very nice add-on.

Although I don't use D&D or any derivative of it, I used parts and concept of it for one of my savage worlds campaigns. That proves how flexible this is. Along with a couple of other, more generic products I think this is a must for any GM.