If you go only by rankings, you are making a decision that is the equivalent of using an opinion poll.
Most rankings are questionable on a minute detail basis, especially if you are trying to determine which school should be ranked 8th or 9th, or close rankings.
However, reputable ones, are good indicators of schools and majors in aggregate. There is a difference between a top 20 school and top 40, just like there is a difference between a top 50 versus bottom 50. There are always exceptions and mistakes, but these do serve a purpose in helping evaluate and potentially select schools.
For example, not having looked at it very hard, personally, I felt ISU's college of engineering was one of the best nationally (and it is). However, when you start to really look closer, it is very good, in the top 40, but not as good as what many Iowans internal view seems to be of where we would rank for engineering.
I had not looked at it that close prior having one going to study civil engineering. The rankings provide an indicator, but visits backed it up. For example the facilities at A&M for engineering are outstanding. The same apples to UT. There is a reason they have a top 10-15 ranking in this area.
It is like comparing Jack Trice to Kyle Field.
It is very akin to the Iowa secondary eduction system. We have great scores on ACT always ranking in the top two-three. Again, this is good. However, when you take a close look at how we do college preparatory, advanced placement, and other advanced academic areas, in my opinion (based on all the data out there), we are just average.
Overall, we are the best of the average. Again, this is good. It just means that we do not look at what areas could be be doing better in.