Prime is essentially seven dollars per month, so consider the following:
Books - do you currently spend $7.00 or more per month on books on this list (Reading one per month):
http://www.amazon.com/gp/search/ref=...blishmarket-20
Shipping: Do you spend more than $7.00 per month on shipping with Amazon on items that Amazon stocks and ships themselves?
Movies: Do you currently use Netflix or Hulu and might you be willing to lose a little on selection in order to cancel one of those services?
Add up the value you expect from each of these - Maybe you'll read a $6.00 prime book every other month (note this needs to be read on a kindle, it will be downloaded directly onto the device,) and spend about $50.00 per year on shipping. My wife and I do the bulk of our non-grocery shopping online, so the shipping was worth it in of itself, but we also found that cancelling Netflix was pretty easy (note the selection is not as large, but there are some things that Prime has that Netflix hasn't - Little Britain for instance.) My wife just got a kindle, but before that, we hadn't even considered it in our decision - it's now just gravy.