Think about it! Gem's shop has highest multiple gem giving ratio (after mystie's nerfing) Negative effects ain't that awful either.
Mostly bad outcomes though for me, I don't know about everyone else, they may have better luck. I just stick with Sichae's apple shop, even though it takes about 50% of my daily earnings, and hope I get some Gems, the other outcomes aren't too bad either, just the one where he puts a small amount of gold in your satchel, even though you spent a whole lot more than what he gave you!