There was a site (intransit.us) that accumulated program reviews from applying students, though it appears to have recently been moved (now mdphds.org) and still under construction somewhat, with most of the links not working. This was a great resource several years ago, and I'm wondering (for current/future applicants) whether this site will be fully resurrected. Something to keep checking back on, though.
More broadly speaking, I think it really depends on what you want. I really don't think the rankings are helpful in this regard, because one person can have a great experience at a top-ranked place that fits them well, whereas another one can be pretty miserable there for a variety of reasons.
If, and especially if, your research interests are quite narrow/somewhat unique (as mine were when I was applying), I would look into the strengths of the corresponding basic science research and, if it's outside the school of medicine, the feasibility of cutting through the requisite red tape to work there as a combined degree student. It's definitely helpful to have a number of potential PIs that you can see yourself working with at a given institution, since they can move, etc.
That aside, a few more things to consider in terms of how much they matter to you personally:
1) Degree of program coordination, and size. The larger, more established combined degree programs tend to know the ups and downs of the process well from extensive experience and may be able to provide better advising.
The especially well-organized of these will typically (though not always) help you smooth the transitions between medicine and grad school and avoid difficult "re-entries." The one caveat here is that at certain programs, the individual med and grad programs are more independent in their rule-setting and don't submit to the whims of the MD-PhD office, which may result in spending extra time meeting requirements that some may feel are unnecessary for a combined degree student.
I think a combination of the above phenomena, as well as random luck, are manifest in how long students take to graduate.
The better programs may also have structured ways to mix science and medicine throughout the curriculum (e.g. grad seminars during med school, structured opportunities for clinical work during grad school) that can be helpful in keeping the combined perspective, though this is a more minor point.
2) The med curriculum. Is it paternalistic (lots of requirements, especially in basic science years) or hands off (very few, and lots of pass-fail grading during this time). This depends on your learning style, and how structured you like it to be. I have a sense that the latter leads to a more relaxed med school class, though this can be up for debate.
3) Location/stipend amount for location/intangibles. Where do you see yourself happily spending a pretty long portion of your life?
