Somebody mentioned that the lychees they bought weren't tangy. That acidity starts going away the moment they're harvested, so when I buy lychees I always start eating them right away b/c it's already been several days (if not weeks) by the time they show up in stores. When I get them off my trees, they're *too* tart, so I let them sit for a day or two.
My Sweethearts flowered heavily this year, but all the fruit was lost b/c of a heavy infestation of mites. (My first time seeing them.) My Brewster tree had no mite issues at all and has a decent crop. Maybe that's coincidence, or maybe Brewster is more resistant to mites. Time will tell.