Are Series Books More Popular Than Stand-Alones?

I just got a strange comment from a fellow teacher who also reads my work. She prefers my "stand-alones" more than my series books. When I asked her why, she said she hates having to wait for the next installment.

I've often read where people exclaim on blogs that they will sometimes wait until a series is complete before starting on book one. I wonder how they can do this if the author doesn't have any idea how many books will be in the series.

I'm discovering a lot of readers are very adamant about saying "yes" or "no" to series books. Personally, I have no preference either way. But I do know that if I read a stand-alone that I dearly love, I'll wish the author could come up with a sequel.