Well, I’ve never used one of the “professional” units… but as they are pretty darn expensive we built our own homemade version and it works like a charm….

It was made out of 12″ aluminum cake pan, and various flourescent electronic parts so that it is both flicker-free and dimmable down to about 50%. We buy plain ol’ cheap circular tubes from Home Depot which are readily available in various color temperatures.
I haven’t used it for any exteriors, but I suspect it wouldn’t be bright enough to do much good. It works great for interiors though, especially at about 5-6 feet or closer. Sometimes I will light a scene so it looks as good as possible, then add the ring just as a “beauty light” to give talent a bit of a punch and give a nice eye sparkle.
The only downside to it I have found is that I cannot use it with any talent that wears eyeglasses… the circular reflections look very weird indeed. You can also get the circular reflections in plain naked eyeballs in extreme EXTREME closeups, but it is usually not much of a problem.
Todd