I'm an event shooter, so I use flash all of the time in situations you are talking about. My criteria for a modifier is as follows (kind of in priority order):
- HIGHLY portable. Needs to be able to move around everywhere I go without slowing me down or getting in the way, plus it must be rapidly adjustable.
- Effective. For me that means it softens the direct light, provides diffuse lighting to boost background lighting, and it must provide light to the full frame of the image (no obvious fall-off at the bottom of the frame).
- Can't look ridiculous. To me the Lightsphere is pushing the envelope...if I am going to walk around with large tupperware on my flash it damn well better be worth it. This isn't a pride thing...I need to look professional, so pie plates, and portable bounce walls just won't do.
- Flexible. I may not always want direct light...may want to bounce from only above or behind me. I don't want a modifier that requires me to disassemble things every time I need to get creative.
I have a plethora of flash modifiers collecting dust and the one I have consistently stuck with for the past 4 years or so is the Demb Flip-it. It allows me to adjust the ratio of ceiling bounce to foward light, effectively increases the relative size of the lights source by 4x over the bare flash, provides very wide-spread lighting coverage with no fall-off, and it's very portable. I even use it outside because of the size increase and because it eliminates the light fall-off that bare flash can give you.
For comparing the Sto-Fen to Lightsphere, they are essentially doing the exact same thing. Both are designed to throw light everywhere, and if you're indoors that can lead to reflections that soften shadows. The Lightsphere will give you a larger relative light source and probably does a better job with the light distribution, but not by a lot. Both are totally useless outdoors by the way. The exception to that is having the lightsphere with a dome on top pointing straight forward -- that only gives you a little larger relative light source. I do occasionally use the Sto-Fen if I am in a tight crowd or if I am shooting into an umbrella/softbox to get a wider spread of light. To me, the Sto-Fen is close enough to the Lightsphere that the extra size and tupperware looks just aren't worth it.
Neuro's setup is no doubt better than any of the about as far as quality of light, but I would never consider mounting the to my on-camera flash. I own one and don't even feel confident mounting to a flash on a stand because it's heavy and forward on the flash. One rapid turn and I can't imagine it flying off into a crowd of people. If you've made that work for you, I'm impressed, but I wouldn't be so brave.
The people who are talking about external lights on stands with umbrellas and such don't shoot events. Great if you want to make a portrait station, but worst idea ever if you plan to be portable. I actually do use that setup when I am shooting on location portraits (like posed wedding shots and such), but once I am shooting candids, everything has to be on camera.
Since someone brought up flash cords, I'll give a big endorsement to third party brands if only because they can be so much cheaper. For instance, B&H makes a brand called Vello and you can get their off-camera cords for about $15 (pretty much identical build quality as far as I can tell). The Canon version is about $70. That means you can get almost 5 Vello cables for the price of the Canon version. If the Canon cords lasted forever, I would say stick with the name brand. The problem is they don't...at all...mine last about a year. Maybe the Canon cable will last longer than the Vello, but it certainly won't last longer than 5 of the Vellos. Plus the Vello brand comes in a 1.5ft model, which is perfect for a flash bracket.
Since I'm plugging gear...ProMediaGear brackets are awesome!
