I've been doing some experimentation lately with things like rolled fondant, a no-cook variation of buttercream, and cream cheese frosting. All of these things use powdered sugar (aka icing sugar, confectioner's sugar, 10X sugar). I've always operated under the belief that for the most part, sugar is sugar. I'm generally more apt to buy store-brand than name-brand sugar, because while it may only be a few center cheaper per package, I tend to buy it in bulk. I go through a lot of sugar. Some may argue that cane sugar is vastly superior to beet sugar, and that's not something I can get into, since I rarely see anything in the store that doesn't specify that it is, indeed, cane sugar.
The fondant that I've been making lately tastes okay, but there's always been something about it that I didn't really care for. It's still been tasting like powdered sugar. I don't know how else to describe it. My best guess was that the sugar granules were still a little big. None of these recipes require cooking (though the fondant does require some warming, and not to the sugar), so there's no chance for the sugar to dissolve. I remember Art Pollard at Amano Chocolate telling me once that he needs to grind his chocolate to smaller than 20 microns, because anything larger will taste gritty on the tongue. Perhaps this was the case with powdered sugar in general, I wondered.
Last night I needed to make another batch of rolled fondant. The store that I went to didn't carry their own store brand of powdered sugar, so I just went with the popular name brand. Sugar is sugar, right? Maybe not. The batch of fondant that I made was identical in measurements and procedure to the previous batch, but the taste was far superior. It didn't have that "powdered sugar taste".
Later on that night, I found a bag of store-brand powdered sugar and used it to make cream cheese frosting. I used to make this stuff all the time when I was a baker, and we always used the name-brand sugar because that's what our supplier carried. The stuff I made last night tasted different. It was grittier, and again, had that "powdered sugar taste".
It seems apparent to me that in the case of powdered sugar, the popular name brand really is a superior product. It may cost an extra 20 to 30 cents a bag, and when you buy cases of 20 bags as I occassionally do, that can really add up. But if you're trying to produce a superior product, then that's a cost that you probably want to just deal with.