This is my name and I've always pronounced it Ree-Ann though it annoys me how when people look at the spelling many pronounce it Ryan and almost everybody I know assumes it's just short for Rhiannon.
My niece is called Rhian and I always say it like RHI-ANN but my sister in law says it's pronounced RHI-IN, sounds like rain when you say it like that. I like it RHI-ANN the longer way.
I am Welsh and I, like everyone else in the United Kingdom, pronounce it ree-un, although in really Welsh-speaking communities Sforzando is right, but generally in British English at least, we pronounce it ree-un. And I've never really liked this name; it reminds me of the French word rien, which means "nothing". :\