I've had the Lightsphere II for a couple of months now. It works wonderfully for distributing a nice, soft, even light to the subject. Now, I don't shoot weddings (you'd have to point a gun at my head to get me to do that) but for the on-location stuff that I do, it works like a charm.
The pros: Nice light, evenly distributed to the subject. Gives a "studio look" without the hassle of dragging lighting equipment around. If you're shooting in a fluid environment, this can be invaluable. Extremely portable and cheap ($40).
The cons: Robs you of about a stop-and-a-half of light, doesn't fit well into the camera bag (that's where his "fits over any lens" hype fits in - everybody was complaining it didn't fit in the bag) and the little rubber gasket tends to get lost. The gasket is provided by GF to ensure a tight fit with your particular flash head - I'm planning on super-gluing mine onto the Lightsphere II once I find the time.