I will basically agree with what others have said. I have not used the Whale Tail, but I have used the Fong Lightspheres for years for wedding photography. His diffusers can do a great job, but again, you do need to experiment with camera and flash settings for the best results.
However, NO on camera flash diffuser is EVER going to equal true studio strobes and large softboxes / umbrellas. With the diffuser, you are still dealing with ONE small light source with a head on direction. The diffuser mimics a larger light source to an extent, but it cannot duplicate a large umbrella and a FAR more powerful strobe light.
Light DIRECTION is as important as power or diffusion. With studio strobes you can use broad or narrow directional lighting to bring out definition of a face. You can never create a true professional look to your portraits with a single on camera light blasting your subject straight on. Your subject will always look "flat" and uninteresting.
The Fong style diffusers are great for events when studio style lighting is impractical, but if you are desiring a true studio lighting look,... then there is NO substitute for PROPER studio lighting.
Steve
http://www.pbase.com/s_parrott