How Important is Job Happiness?

I frequently talk to founders who left their corporate job to run their startup full-time. But what about those who don't have that million dollar idea (or want one!) but aren't happy working at a startup or a corporate company? My advice is simple: we all have our own definition of happiness, but how much are you willing to sacrifice to a) land your dream job b) stay in your existing job because the money is good even if you're not happy? When is too much unhappiness enough to get you to pursue something else?