What makes a Product Good?

[DragonLancer]

To me best and most useful go hand in hand. IMO the environment series and most monster sourcebooks are in that category because they are useful for any campaign, and almost anything that comes to mind.

Books such as the Complete Guides, PHB2 et al, are not useful nor best, because they just pile on more and more unnessecary options.

 

[DaveMage]

This is an easy one:

A product is good if it both inspires me to play the game, and use that product in-game.