As "ß" vs. "ss" changes pronunciation of preceding vowels, I can't see how it could be anything other than its own letter.

* "Fuß" ("foot") roughly rhymes with "loose."

* "Fluss" ("river") roughly rhymes with… um, nothing I can think of. It has the vowel sound of "look" and "book," at least as pronounced in the American Northeast.

Since the orthographic reform of 1996, this has become a big deal.