Well... I day Ree-sees when I am just talking about it. But when I am talking about Reese's Pieces, I say whatever pops up first, usually Ree-sees.
The only reason I'm not sure about the pronunciation is simply because "Ree-sis pieces" rhymes, otherwise, I'd probably say "Ree-sees"
I say "Ree-sis", but that may be because of my British accent, I dunno. I pronounce grass "Gr-arse", bath, "B-arth", etc.