I did my M.S. with the plans on going for a Ph.D., although I always considered medicine, I figured Ph.D. was an easier route, and I'd enjoy the lifestyle more.
When I got to the lab, it lacked people, other than the other scientists. Its a very tedious job, and fairly lackluster IMO. I'm glad I got so much research experience, and I'm sure it'll be useful, but its just not for me. Keep in mind that most of the time undergraduate research is not real research. Most of the time you're given experiments that are nearly guaranteed to work with defined protocols. Most of the time you're tying up loose ends that a graduate student didn't get around to.
Although I love my PI to death, we're on severely limited funding. I basically survived on scrap materials, and managed to put together a very interesting thesis. Had I more funding and time, I think I would have a fairly significant paper to publish.
And thats what it comes down to. In academic research you live and die with funding... not a way I want to live my life.
Another note is that people think a medical graduate can go in to research. This is wrong unless you already have strong research experience. Many of the medical students and MDs we collaborate with really don't know which end of a pipette to use, they're inexperienced. There are opportunities for MDs to do research, but don't think you won't have to pay your dues in the lab.
Right now the NIH has a hard-on for Physician Scientists, and they're having an easier time finding funding, but its still tough. This environment can change with the economy, or an anti-science president.
To summarize: Being a academic scientist is about uncertainty. You don't know if your experiments will pay off. You don't know if your findings will be significant. You don't know if you'll be able to secure funding. If you're okay with this, and comfortable with the idea of falling back on private industry, then go for it.
This is just my 2 years as a graduate student speaking, and my situation is likely entirely different from others.