If you are really serious, get the one made by Profoto. Big bucks but it is awesome. I use it all the time and it is a wonderful lighting tool. Get Profoto Acute 2 powerpacks (1200ws is a good size) and the ringflash head and the big reflector. This is a standard item with all the Playboy in-house photographers.
Here's a link:
Once you rent, buy or use Profoto strobes you will then see why it is the most rented professional strobe equipment in the world. I sure have spent enough money with them! It's the best ringflash I have ever used.
Now, for using it. The look it gives you is obvious and you will love it. The Profoto head can take a LOT of power so you can use this with or without the big reflector outside at big distances from the model and dump up to 2400ws into it if you need to. I usually use it with the reflector for shooting girls but sometimes when shooting CD covers for "hip hop" acts, where I want an "edgier" look, I'll take the reflector off and go with a "harder" light.
This is a very tricky light to meter because it is on the camera and as you move in relation to the model, you are constantly changing the light level. Remember the "inverse square law" of light intensity. If you double the distance from the subject you have reduced the light by 1/4. If you move in twice as close you just increased the light 4x. Digital camera bodies are really handy here to see what you are doing in the LCD screen! I move a lot when I shoot girls, so I am always having to monitor and adjust my ever changing exposure when using a ringflash.
But it is a one of a kind look!