"Lychee" is one of those flavors that I couldn't describe to you, but I'd know it if I tasted it. With that "knowledge" (or lack there-of), I cracked into this earlier today before class:
I didn't know anything about Sprecher's "Diamond" line, but I found these fancier sodas in a Reddit thread:
Brand: Sprecher Diamond Lychee Soda.
Origin: Glendale, WI.
Purchase Place: Blue Sun Soda, Spring Lake Park, MN.
Sweetener: Sugar, Sucrose, and Raw Wisconsin Honey.
Review: This was sweet and smooth. There's a slight fruitiness to it that makes it quite drinkable. The "lychee" flavor is what I'd expected (sorry, I can't define that flavor well, but THIS was IT). There was a slight harshness in it's aftertaste, but that's the only negative thing I can say. I'd be excited to find those other 3 "diamond" sodas shown above.
Oh, and being it's Sprecher, I'm not surprised that honey is one of it's sweeteners. They use honey a LOT (from what I can recall). This had THREE sweeteners, so it was good and sweet, but not TOO sweet.
Score: 8 out of 10.