The Appeal: It's exciting to think you'll have a life of problem solving and fascinating experiments that will help humankind live longer and healthier. Then there's the prestige: It feels good to tell people you're a medical scientist.
The Reality: You may have a better chance of winning the lottery than of making a significant medical discovery. First, you must have the superlative undergraduate science and math grades and GRE scores to get into a first-tier science Ph.D. program, plus the ability and perseverance to complete it and, usually, a postdoctoral fellowship. Even if you do all that, the odds of landing a good research job are modest. Even more discouraging, only a small percentage of medical researchers make even one significant discovery in their entire lifetime.
Then there's the quality of life. You spend most of your 60-to-70-hour workweek alone in a lab or at your desk, with little people contact. And the pay? According to MIT faculty member Philip Greenspun, "Adjusted for IQ, quantitative skills, and working hours, jobs in science are the lowest paid in the United States.... [You spend] 10 years banging your head against an equation-filled blackboard in hopes of landing a $35,000/year post-doc job" and then in hopes of two years later getting that rare $70,000-a-year job as a tenure-track professor or a somewhat less rare position at a pharmaceutical company.
Alternative: Medical Librarian. Most people choose to become medical researchers because they'd enjoy solving science puzzles that could lead to medical discoveries. Your chances of doing that are probably greater as a medical librarian because every day, whether you're employed at a university, a hospital, or a pharmaceutical company, you're solving lots of people's problems by unearthing the resources they need. Bonus: Each request exposes you to some new aspect of medicine—unlike as a researcher, where you're probably plowing away at similar things for years. And the training is shorter and easier to complete: A career in medical librarianship requires only a master's in library science. Further boosting your odds of success, it's an under-the-radar career, so you'll have less competition for job openings.
More info: Medical Library Association.