I think in most cases they are highly ovrerrated. Someone above me posted a decent list of when to use them.
Remember, a good mine or mill is going to get you about the same increase in production if the city is covered by another industrial zone.
1st question you should ask is, 'what will be my return on this investment?'
I factor in whether or not I will be slotting citizens there.
Their value goes up for a science victory.
As mentioned elsewhere several times, if you get a double tap on that great engineer, they are worth building in almost every city through mid-game because the factory ends up being another +4 even if already covered by another zone. So +6 with workshop, + adjacency, + citizens.
I've built them just for engineer points.
Another factor is if you are getting the % bonus to production from faith.
It could be considered counter-intuitive, but the more dom you go, the less value they have. Imo of course. And the further into a game you are, assuming victory conditions are involved, the less value they have.
But I think most would agree that you want at least a couple well-placed ones in most games.