The always notable Seth Godin:
" 'Never Quit.' What a spectacularly bad piece of advice. I think the advice giver meant to say 'Never quit something with great long-term potential just because you can’t deal with the stress of the moment'. Now that’s good advice."
"It’s time to quit when you secretly realize you’ve been settling for mediocrity all along. It’s time to quit when the things you’re measuring aren’t improving, and you can’t find anything better to measure..... Smart quitters understand the idea of opportunity cost."
"What’s the worst time to quit? When the pain is the greatest. Decisions made during great pain are rarely good decisions."