My verdict: I put them in a brown paper bag with a succession of fruits (bananas, apples, etc.) to try to encourage ethylene-dependent ripening. After a month I tried the second one because it was quite soft (about as soft as a ripe mango). It still made my mouth feel hairy. So I waited another two weeks until the fruit looked like it had gone bad. It was shriveled and mushy. But it was finally ripe. It tasted like a sweet potato with a tangy aftertaste and didn't make my mouth feel funny. Nonetheless, the whole ordeal still left a sour taste in my mouth. I'd rather eat fuyu persimmons since they don't require another month and a half to ripen after purchase. And if I want the taste of sweet potatoes I'll just buy a sweet potato.
 Read my review of fuyu persimmons here.