I am a senior cardiology fellow going on to an interventional fellowship - so consider my biases as you proceed.
1) Where government is the biggest payer - reimbursement will ALWAYS go down - cash business (Lasix, Plastics, etc) will survive health care reform because they do not depend on Medicare/aid payments. I am doing cards cause I LOVE going to work every day - but show me a cardiologist in 10 years making a million a year - probably doing something shady - its gonna change and cardiology is a major target of health care reform (the cardiologists of the 80s and 90s screwed the pooch royally for us)
2) Yes you do have to be smart to be a cardiologist (and an opthalmologist, and a GI, and a Rheum, etc) - whoever posted otherwise is mistaken. That was just a dumb comment.
3) GI does not have the "lifesaving" satisfaction to the extent that a poster above suggested - if you require that satisfaction, only Cardiology and PCCM( (of the medical subspecialties) enjoy that satisfaction as a matter of bread and butter. The GI fellowship at the 2 institutions I trained worked waaaaay harder than the cards fellows - FYI.
4) I suspect many people feel the same, but I went into medicine knowing I was gonna work hard, but that everyday would be a change of scenery and have meaning - it was the opposite of cubicle life - so consider that 80hrs doing something you love, can seem much shorter than 40hrs doing something you hate.
5) Just my personal experience - the reasons I said I liked fields in my senior year of med school, turned out to all be B.S. Be honest with yourself, and don't let the tree-huggers/Koom-baya (sp?) MDs of the world make you feel bad for considering, income, prestige, bad-ass-edness, etc, they shouldn't be sole motivators, but they are not wrong. I personally needed to be in a field where I can come in and save the day - that was important to me, therefore, Geriatrics - a very noble and necessary field (my dad and sis) - wasn't gonna cut it for me. Good luck to all you students figuring this all out.
6) Cardiology is best field EVER!