I use sto-fen as well as Joe Demb pro diffuser. It's basically a bounce card and diffuser combo. It is very easy to use and more adjustable on the fly than I feel a fong sphere would be (although I've never owned one). Couple of things I like about it, universal connection - fits any flash, adjustability - card is easily moved to adjust how much light you throw forward and how much gets thrown up for a bounce. Check 'em out. I found it to be very good investment. Also Joe is great to work with on the customer service front...
Demb Flash Products - Demb Flash Diffuser Models