Sweeteners taste disgusting and leave an aftertaste that lasts all night.
Well this obviously isn't a fact and it's your unfortunate experience, myself and my family don't seem to have these issues. I think people just like being over dramatic about this stuff because they don't like change.
I can't speak for your own experiences, but I think that, as a nation, we've become so used to artificial and over-processed food and drink that we've forgotten what natural food actually tastes like. If you are used to more unprocessed food, the artificial stuff comes as a real shock to your system.
Possibly not so much the case with Stevia, but artificial sweetener tricks your body into thinking (and reacting appropriately) that you're taking in a load of sugar. For people with diabetes, this can cause very adverse consequences. How is it good for anybody tricking your own body?
I don't object to sweeteners, if that's what people want - I just don't understand why they can't continue to make both varieties and charge more for the sugary one to include the sugar/anti-diabetic tax. Who is there out there wanting a diet/lo-cal/sugar-free version, but seeing one with only half as much sugar in as well as the sweeteners and deciding that's the one they want after all?
They wouldn't dream of doing it with alcohol - sell what appears to be normal lemonade, but just pop a bit of alcohol in there; they wouldn't sell 'sausages' containing 100% pork and also offer 'vegetarian sausages' containing 'only' 50% pork.
Donald Rumsfeld has become a phenomenally wealthy man because of Aspartame and it's far from all good if you look into it.