The poll closed last Friday, and with about 300 votes, here were the top five:

 Journal of Philosophy was the runner-up for the top five.  Some journals that run extensive and valuable review sections, like Journal of the History of Philosophy, were not included since they don't run reviews across a range of areas, contemporary and historical.  There were some accidental omissions from the survey, like Philosophy in Review, though I think it doubtful  any of the omissions would have displaced NDPR.  NDPR's success is due, I would venture, to the general quality of the reviews, their substantive character (due to fairly generous word limits), and their timeliness.  (I should note, of course, that I am a member of the NDPR editorial board and have contributed several reviews there.)  Gary Gutting (Notre Dame) deserves credit and gratitude for conceiving and managing this project so effectively over the last decade.  

UPDATE:  Steve Gross (Johns Hopkins) writes:

Perhaps a further reason for NDPR's success is that many folks subscribe to their email service. Speaking for myself, I'm more likely to read NDPR reviews simply because I don't need to go get them. And, if I find myself with a few minutes downtime with nothing to do (waiting in a line, say), I'll just take out my phone and read an NDPR review.

That is probably right, and was part of the wisdom of creating an on-line review service.