I'm inclined to believe this is due to gut flora, as described in this article:
I found a studying demonstrating similar gut flora alterations in both rats & humans:
It's behind a paywall, but you can still view the extract. The authors have also linked the regular consumption of artificial sweeteners to glucose intolerance, but I'd have to read the whole study before I'm convinced.
I've heard Stevia can have a bitter, licorice-like taste if you use too much of it, but I haven't used it enough to confirm this.
As other have already said, I suppose the best answer to the artificial sweetener dilemma is "always in moderation."
But let's be honest - artificial sweeteners are added to supplements purely for marketing purposes. Personally, I couldn't care less how a supplement tastes so long as I'm getting the physiological benefits, but this may not be a common opinion.
I think Brain Candy is a fantastic energy supplement, even without the added caffeine, but since it has artificial sweeteners, I'm hesitant to use it often, which means I'm not going to buy it as often.