The qustion is; Are they worth it?
Well, so far, I only have one, and I got it for 500 Gems (because I'm a payer-player and had the gems). Why did I buy it? Simply because the mage can cast a spell once per 24 hours that speeds up one action in your city.
You can use him to speed up a breeding in the Breeding Sanctuary, the tree, or the mountain.
(You can use him to speed up a building, though I haven't tested it out yet, I imagine you could) < Untested theory!
You can also use his spell to speed up hatching. HOWEVER when speeding up a hatching in your hatchery he ONLY speeds up the egg with the least amount of time left. So you can't aim it at a particular egg. That might be a negative to some, but I don't have a problem with it. I can just manage to free up another nest in my hatchery for a new egg.
Since I've purchased him, I've used it every day. Typically for speeding up a breeding. I figured out the hatchery thing as well, noting that his spell does not cover the entire hatchery, merely the egg that has the shortest amount of time left to hatch.
I've added a slideshow so you can see what happens.