I have also left Things once. And tried many of the mentioned alternatives. And many others not mentioned here. And then I returned. And I immediately felt "at home" again. Some features missing, very few are really important, if at all. For those who wish to experiment and lose time, go ahead and try all the other stuff. For those who wish to work, effectively, I strongly recommend Things. Oh yes, and don't forget about the monthly fees, some are really not that low at all. With Things you pay once, and go for a life-time.