According to From Flappers to Rappers: A Study of American Youth Slang by Dr. Thomas Dalzell, "the berries" was a 1920s widely used slang term among American youth to describe something wonderful or very good. I shouldn't be surprised if etymology reveals "the berries" to be much older than the 1920s; however, what I'm most curious to know is why anyone would describe something wonderful or very good as "the berries". Any thoughts?
I don't know that Dr. Thomas Dalzell and I would have the same definition of "widely used slang", but I more than suspect this use of the berries originated in an 1869 Punch cartoon...
I see The New York Times reproduced that cartoon in 1911 (search for "berries" on that page, and it's in the OCR text). The (comparatively few) written instances I've been able to find suggest the term was mainly associated with wealthy young people (whose parents probably read NYT). In Proceedings of the Royal Society of Medicine, 1924, the use of "scare quotes" suggests to me that particular doctor expected at least some of his upper-middle-class readership to appreciate the reference (i.e. - it may have been something of "catch-phrase" for a certain class of people).
It seems to me if there's any meaning at all to the search for an "origin" of such a short-lived and geographically/socially constrained usage, it's more likely to have arrived via the Punch -> NYT route than from a more "earthy" allusion to testicles.