Sealed whey protein container past its use by date - is it still usable?
You didn't say where your product was packaged, but according to the USDA, "Use by" dates refer to best quality, and are not safety dates.  This is especially true when foods are handled well...in your case, factory sealed.  Additionally, I can't see how the proteins would go anywhere.  Given the reportedly good flavor (not an indication of safety, but simply of good taste), I would use it.