Above is Amazon's breakdown of what Prime offers, and to be honest it's all right there. I don't really understand the "Instant Access to Kindle Books" considering they're always instant access once you buy them..
Here's my breakdown, if you fit the bill, Prime WILL save you money. If not, well then it's up to you if you want to shell out the $79/yr. $39/yr, if you're a student.
Do you order more than 10 times a year?
Are you impatient?
Would you like free movies with your free shipping?
If you answered yes to any or all of the above questions, you should get prime and never look back.