When I worked at arXiv I looked at usage statistics that we didn't make public because we didn't want people to get the wrong idea.
One thing we knew is exactly that: the most viewed papers were review papers. You read a lot of them on the road to a PhD.
Another strange thing about review papers is that they escape the usual standards for evaluation in science. That is, as an outsider I can appoint myself to write a review paper without doing any research in the field, and it's possible I could do a very good job. One of the fun things I did in grad school was make a bibliography and short review of papers on the phenomenon of "Giant Magnetoresistance" at the request of the experimentalist on my committee.
