For about a year I shot portraits only with available light. My office/studio has big north-facing windows that give an excellent soft light. The problem was that I could only book shoots in the afternoon, which meant mostly on weekends. And I had to cancel if it was not a sunny day, a frequent occurence here in San Francisco.
I've just recently started using a softbox with six 80-watt color-adjusted compact flourescent bulbs as my main light source. I totally love it! It gives me the broad soft light that I like so much and I can shoot anytime, anywhere!