Hey JulianCrane,
first of all, I would recommend you read through the guide and FAQ at
http://intransit.us/. There is just no better way to get a general idea about the process. While I couldn't possibly do it justice in this post, I short, an MD/PhD program is a combined degree program that trains people to be physician scientists. What a physician scientist actually does is a little harder to explain. While it varies, it usually means a person in academic medicine who does clinically relevant research (basic or translational from a clinical perspective or clinical itself), and/or practices medicine.
Training takes 7 to 8 years, with first two years being the first two years of med school, followed by 3-4 years of graduate school and dissertation research, and finally two years of clinical rotations (notice how typical medschool curriculum is separated by PhD work). This is most often followed by a standard residency (3-7 yrs) and sometimes a postdoc (1-3 yrs).
Clearly it's a long process, but it is generally thought that MD/PhDs emerge extremely well trained, and are therefore quite highly sought after. Since most stay in academia, however, over time they will earn less than their private practice clinical counterparts, which is one of the reasons that the whole process is funded (tuition waiver, health insurance + ~$20,000 yr. stipend) to avoid debt.
It's not for the weak of faith. I dare to say it's one of the most rigorous educational experiences one can choose to pursue. The 'free' education or even desire to do 'some' research are simply not worth the time and the effort needed.
If you are interested in doing some research, but not quite willing to make such a commitment, I would suggest you look into MD programs with research requirements, such as one at Yale, where you take an additional (tuition free) year to do some research and write a thesis (not a PhD).
If the idea of pursuing the rigorous research and a lifetime in academia seem like what you want to do with your life, then the MD/PhD program is the best thing since sliced bread.
I always opt to present what I see as the brutal truth when people who are not sure ask about MD/PhD. I don't think it's something you should be talked into. I feel that one should go into it if, and only if, after hearing all the reasons NOT to go into it and attempts to dissuade you - it is still the only thing in the world you see yourself doing.
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As far as the programs go - they are all very competitive (generally speaking more so than MD or PhD alone). They also look for slightly different things, primarily research. Also, note that it is the independent and creative research they are interested in.
I can only comment on WashU's program from personal experience. It is probably the largest, among the best funded and well organized programs in the nation. Definitely the most competitive of the three, and probably one of the most competitive overall. They're huge on research, and they like high numbers, as well. It follows the pattern described above, and you will get unparalleled research opportunities (especially in neuroscience and immunology). There are really very few things I don't like about it, the main one being the location (looks like it's going to be a breaking point for me).
Anyway, again, check the link above (also includes program reviews), do some soul-searching and ask us more questions.
Good luck.
