I don't own one but I shoot with photographers that use them.
Personally, I'm not a big fan. Sure, it's great when you use it in situations that are ideal for it...small rooms with nice bounce surfaces....but I often see photographers using them in situations that don't make a lot of sense to me.
The problem is that it throws light out in all directions. So when you are in a large room without good bounce surfaces or outdoors...it wastes all the light that isn't going right to the subject.
Also, many people seem to think that by simply putting a translucent material in front of a light, the light will get softer (ie better)...and that simply isn't true. The only way to make light softer, is to enlarge the source or move it closer to the subject. Sure, the 'Fong Dong' does increase the size of the light source over a bare flash head...but only slightly. If your subjects are more than a couple feet away, the size difference is too small to make any real difference in the light quality....plus it's still wasting all that light and the translucent material eats up some of it as well.