I left MIT one year ago - I had one year of coursework left as an undergrad in Aerospace Engineering. After [a long story] I co-founded segment.io with @ivolo, @ianstormtaylor and @calvinfo. I'm incredibly happy with my decision, and I want to share some reasons why the decision to leave was right for me.
First: my family, and especially my fiancee(!) have been incredibly supportive. It's WAY EASIER when they smile, listen and help you through the rough times of starting a company. I'd have given up long ago if not for them. Not to say it was a perfectly smooth ride: on first meeting Erika's parents, I told them "I'm gonna give this entrepreneurship thing a try." Her parents deserve some serious credit... I'm surprised the conversation didn't end in laughter right there.
My favorite professors have mostly gotten over their shock. I was back for MIT's graduation ceremony last week and got to say hi to a few of them. They were mildly surprised to see that I was not, in fact, broke, hungry, and homeless. Our progress delighted them, although they expect me to finish up at MIT eventually. No worries, these profs aren't going anywhere.
It's also fascinating to observe the difference between leaving school with purpose (let's start a company!!), and getting suddenly shoved out of school right after this odd thing called "graduation". Today I live with my co-founders on Russian Hill in San Francisco. It's a grand thing, living with your college roommates in an amazing apartment, managing your own time and working on something you chose out of your own interest.
At the same time, many of our friends from MIT/RISD and high school are now suddenly a part of the adult working world. And whoa is it a shock. A rare few of them are overjoyed in their realizations "Wow! I'm paid so much I can buy a house, like, right now!", but many of them are now soul-searching for a job that actually fits their credentials. Some are re-training in radically different fields (humanities --> computer science) and others are going back to school in... oh, computer science. This time around though, they're training with intent. Leaving school with a purpose feels a lot better than leaving without a purpose.
So, while I don't have any reason to specifically recommend leaving school early, I do recommend leaving school with a purpose. For me that happened a year early.