What do you want to do when you "grow up"? If you are hanging out on HN, I assume you have some interest in start ups...read my other comment. And I can give you a lot more reasons why you should continue [especially if you are at one of the better institutions].
The best thing to do if you are facing the option of a PhD and also interested in other paths (startup, bigco, etc) is to fully understand what the PhD will do for you and what it is like. Including the impact it might have on your mental health, career, and ability to do a startup later. Reading essays like pgbovine's are a great way to fully understand this decision.