The primary interest in the C-trumpet I think is the ease in transposition (marginal, but its there) and its tone. I personally prefer my C to my Bb
Piccolo takes some getting used to, but once you do, I don't think the back-pressure is bad (you're probably overblowing, because its a very different horn) The range doesn't go as low as a "normal" trumpet but the upper register is easier. Generally not additional range, but its easier.
(And i've only ever had problems with my eyes while playing a Bb
