In my experience... if you got an Mexican alt. color (white body with purple hair) Sundae Best pony that is not gooey, you'll have an amazing thing in your collection.
Sadly Mexicans seem to be prone to plastic degradation, specially the ones with white/cream body color (Newborn Babies Rattles and Tattles), the alt. Sundae Best, the pink Perfume Puff pony, and the princes (and my Mexican Brady the Dog). Amazingly (in my experience) both Moondancer and baby Moondancer doesn't suffer this condition, Starshine is a weird case, she mostly suffers from missing symbol but not gooey body.