I've used Gary Fong's dome, Stofen diffusers, gamilight 21, gamilight event pro and a host of other diffusers. At the end, I find that I reach out for the Stoffen diffuser caps for all my speedlights.
Gary Fong - good in large rooms where ambient light is sufficient. Great for large group portraits when you have certain control over the whole scenario. Power hungry as your flash tends to output at or close to max power all the time. Recycle time will be longer. Also, it's heavier and quite cumbersome to wield around all day long. I used the 2nd generation which wasn't collapsible so the weight and protrusion was even worse.
Gamilight 21 - probably the best soft light effect of the lot. In a scenario where you don't have to rush around to get the shots, these are perfect. I use it in black tie occasions where people are more civilized about posing for you. Make sure the flash you are using has TTL flash and great AF erformance in low light as the diffuser will be blocking the AF assist beam from the flash infrared.
Gamilight Event Pro - I use this where subject matters more than anything else, including overall photograph. This set up provides the most efficient use of battery and the most unrestricted light to the subject. In good ambient light, this is almost ideal. It does protrude so watch out for obstacles.
Stoffen - Great all rounder and robust. Quick and difficult to knock out of place. Only thing about stofen is that it doesn't spread the light far or wide enough when ambient light is very low. So, ISO balance is important to get that natural looking photo.
Fuss free and practicality, I would recommend Stoffen or Gamilight Event Pro.....or similar. Watch out for stoffen copies. Some may cause a slight color shift.
Good luck.
| 
