I'm in the same boat, been mulling over pursuing med school for the past 6 months or so. I had *never* considered it before, so I went in with wide eyes.
Reading books, talking with people, and obsessively absorbing studentdoctor.net were all helpful - but the most insightful has been volunteering in a hospital. At first it was exciting (I love hospitals and airports, hustle and bustle) - I love seeing behind the curtain.
But then I started to see the routines behind everything - and once that "new car smell" faded away, you're left with a high stress job that doesn't really give you much personal freedom. Ever have those brain fart days when you just waste time chatting with your coworkers? There has to be some specialty where you can get away with this, but with most docs the job seems to fully consume you. I like the chance to be a bit lazy, let the creative juices flow.
I'm a sucker for the glamor of med school and residency - I like learning new things, being pushed in training. But once you get out - I've finally seen (as most of the residents/attendings seem to preach) that it eventually becomes "just a job". Sure you get to help people, but I think any buzz this provides wears off fairly quickly in residency.
Obviously, this is all highly subjective, just throwing out there all the things I've learned by getting out and experiencing "the system". If you're thinking about this - get involved somehow so you can actually see what it's like. You'll either slowly burn off your excitement, as I'm managing to do... or you'll get pumped about everything, and get all fired up for the MCAT and applications.
Even if I never end up pursuing med school further - what I've learned so far has given me a *much* deeper respect for docs, and all the BS they have to endure. I'm amazed anybody goes to med school - but I'm thankful there are so many hardworking insomniacs out there with an unlimited tolerance for pain!!