Here's a trailer running down some (but not all!) of the many Street Fighter IV costumes to date, which will be included in next month's retail release of Ultra Street Fighter IV.
Come for the costumes, stay for the cheesy song. Balrog's American boxing robe alt. and Mech-Zangief are my favorites. Though I've made plenty of regretful cosmetic DLC purchases in my time, I never did end up buying any of the Street Fighter packs.
USFIV Costume Trailer [Capcom-Unity]