One MS employee recently said to Stephanie (my partner) that (paraphrasing) "your company isn't stable and can't possibly last". My reply: We've been in business for over a year now, and our business is just a natural extension and continuation of our careers. I've been programming since 1985, and developing commercial data compression and other software since 1993. I've been doing this for a while and I'm not going to stop anytime soon.
Having my own small consulting company vs. just working full-time for a single corporation is just a natural next step to me. One thing I really liked about working at Valve was the ability to wheel my desk to virtually anywhere in the company and start adding value. I can now "wheel my desk" to anywhere in the world, and the freedom this gives us is amazing.
Binomial is a self-funded startup. We work on both development contracts and our current product (Basis). We haven't taken any investment money. Our "runway" is basically infinite.