[parttimepro]

I'm getting a Ph.D. in neuroscience at a top school. Not sure if you realize this, but graduate school is basically training to become a professor. Some schools make noises about "preparing students for careers outside of academia" but this is basically an add-on. Your classes and day-to-day research are oriented almost entirely towards mastering a small segment of knowledge and adding your pebbles to the mountain of human knowledge.

Career path: If you go for academia, your life looks like this. 4-7 years of graduate school, followed by a postdoc (glorified graduate student) or two of 2-5 years. If you've done good research in a growing field, you'll get an assistant professorship, where you can work your ass off teaching and researching to get tenure in 7 years. This is for the sciences. My understanding is that for the humanities, it's more like 6-8 years grad school, then be a teaching adjunct for an indefinite period. At any point you can drop out and work for someone else in your field (if you're in science) or in the growing field of fast food service (if you're in the humanities).

Lifestyle: It varies a lot. I know some people who claim at least to work 60 hour weeks. Personally I have time to post on 2+2 at 1:50 on a Tuesday. Still, you're going to work a lot, and you need to like the process of research. If you haven't done any research as an undergrad, you may have a hard time getting accepted to grad school.
By far the best part of grad school has been meeting other people who are a lot like me. It's kind of like college, without the dumb people.

Money: You get paid to go to school. Not much, but I can live comfortably and won't graduate with debt. Postdocs make 30-45 depending on experience. Assistant profs start at 60 or so. If you're interested in business, some profs are able to start businesses based on their research, which can be quite rewarding. I believe engineering is generally the best field for this (better than basic sciences).
Alternately, a Ph.D. does prepare you for several other careers. One thing I'm considering is management consulting (a la McKinsey). Some companies just try to hire smart people, regardless of the details of their credentials.