Evidently, over the last decade or so, most diet sodas have been using a blend of two sweeteners--aspartame is still the primary sweetener, but acesulfame potassium (also known as Ace-K) is added as a secondary sweetener. The Wikipedia article on Ace-K says that this blend supposedly allows each sweetener to mask the other's aftertaste.
"Supposedly"? Based on my recent experiences with diet sodas, I believe it, regardless of whether's it's really true or not. All the diet sodas I rated at three or more stars used the aspartame/Ace-K blend (although I don't know if Fresca had switched to that blend before or after the last time I drank it--if I liked it without the Ace-K, I'll like it even more now). All the ones I rated at less than three stars used only aspartame. (All of Faygo's diet sodas use only aspartame--a total shame because one thing I love about Faygo is its variety of flavors.)
Moral of the story: I need to read the ingredients first before I buy any diet soda.
Diet Canada Dry Ginger Ale, Diet Sunkist Orange, Cherry Coke Zero, Pepsi Max and Diet Pepsi with Lime have all gained the "thumbs up" from me as the result of using the aspartame/Ace-K blend instead of just the aspartame. I would buy any of those again, but out of this group, I liked Diet Sunkist Orange the best, perhaps because the orange flavor in it was the strongest (and thus mitigated the aftertaste better than the others). (But just in case you wanted to know, Diet A&W Root Beer is still the Diet Soda Federation World Champion.)
It was after drinking the Diet Sunkist that I got to thinking, Could I be getting used to the aftertaste? My mother told me that she could never switch back to the regular stuff because she found that it tasted too "syrupy." So I asked my Facebook friends about it, and two of them said that yes, they got used to it and couldn't drink regular soda again afterwards. Maybe that's happening to me as well.
It all reminded me of a scene from a 1995 episode of Star Trek: Deep Space Nine in which one alien (Quark, the Ferengi bartender) introduces another alien (Garak, the Cardassian tailor/spy) to a human drink (root beer). Garak reluctantly takes a sip and says, "It's vile!" Quark says, "I know... But do you know what's really frightening? If you drink enough of it, you begin to like it." Garak responds, "It's insidious."
You can find an MP3 file of the root beer scene here.
Video can be found on YouTube as well here. (The root beer scene starts at the 1:50 mark of the video.)