Kimberlee has a good point. You'll never know how "well" students match from a school unless you know what those students wanted.
However, I would say that if you are evaluating a match list, you should look for THREE things: matches in competitive programs (eg. internal medicine at Mass General is obviously more competitive than internal med at Northwest Idaho Community Hospital), numbers of matches in competitive specialty (eg. derm, ENT, etc.), and percentages of first choice matches.
Unfortunately, there are problems with each of these criteria:
(1) For "competitive programs"--it's hard to know what programs are most desirable. USNews doesn't rank residency programs (yet... thankfully). Often you can't judge a residency program by the "brand name" of the university. Also, locations and lifestyles can play a big role in how applicants rank these programs. So what might be attractive to a 4th-year medical student might not be obvious to the pre-med looking over a match list.
(2) For "competetive specialties"--we know which ones are hard to get, but, as kimberlee pointed out, not every medical school class is full of students who want to be dermatologists. So just looking at numbers of matches in competetive specialties can be misleading. If few students want to go into competitive specialities that year, then this statistic is artificially low.
(3) For "percentage getting their first choice"--the problem is the reverse... If 40 students at a medical school apply for psychiatry, a lot more of them will get their first choice than if 40 students apply for neurosurgery. So, if a lot of students want to go into less-competitive specialties, this statistic is artificially high.
Bottom line: do your best to get a feel for how students match, but don't get too bogged down in the details. And don't base your entire decision between two schools on small differences between match lists.