I'm going to be honest with you. Berries aren't as sweet as you think.
Yet when you think berries, you think sweet, largely because you think of what we do to them: crisps, cobblers, cream and sugar. Fresh from the bramble it's a different story, as the flavor profile of a berry is often dominated not by sugar but by pucker.
I generally prefer the taste of unadulterated ingredients, so I appreciate the tang of a real berry, but the fall raspberries I've been eating lately have me singing a sweeter tune. I don't really know what I'm talking about here, but it seems as though there are very different raspberries in summer and in fall. Also, there are raspberries in fall at all.
I usually think of raspberries as a strictly summer thing (barring tasteless - in more ways than one - imports) and am glad to see that they're getting a second wind. My hunch is that the fall raspberry is a different variety that's on the up and up as eating locally and seasonally gains ground.
From what I've observed (and not from any actual research), fall raspberries differ from their summer counterparts in two ways. The color, like the weather, is darker. The flavor is sweeter, more mellow, somewhat honey-like and almost absent of any acidity.
I'm reminded of a passage in the Omnivore's Dilemma about different recipes for eggs from different seasons, and I wouldn't be surprised if, in our recent past, there were similarly varied treatments for summer and fall raspb's.
I'm sure that in the annals of cookery there are absolutely delicious fall raspberry-specific tarts and sauces and such, but they're just so good that I can't stop myself from gobbling them up unaltered.