The CEO who got ousted, in 2011:

“Without outside capital, we have to make do with less ... Constraints are a beautiful thing because they force creativity and precision. We don’t have the resources to throw after hit-or-miss hires or strategies. Bootstrapping a business requires a different mentality. It’s taught us to be frugal, hire slowly, and exercise caution as we grew the business. While companies that take funding can do those things, people have a tendency to behave differently when it’s not their money on the line.”

This was about 1 year before they raised a massive round from NEA/Accel. From "Bootstrapped, profitable, and proud" on 37signals blog: http://37signals.com/svn/posts/2800-bootstrapped-profitable-...