I have two Nikon SB-26's and one Sunpak Super 383 Flash that I use in my Real Estate Photography business. I purchased Sto-Fen diffusers for all of them and have been pleased. They are made with very durable plastic and fit the respective flashes like a glove and don't fall off.
I always describe the light quality with the Sto-Fen installed as "creamy", especially as compared to the hard light coming from the unmodified flash.
For what they are, the price is a bit high. I bought three Slik Tripod with 3-Way Panhead to use as light stands and they were only two bucks more than these diffusers.
TIP: Before buying the Sto-Fens I used the bottom half of a rubbing alcohol bottle as a diffuser on the 383. The light quality was just as good if not a bit better. It's not as professional though, so I went with the Sto-Fen.