...
Edit: Also, you should make sure that what you want to watch is actually included in prime instant video.  I think you still have to pay for the latest myth busters seasons even if you sign up for prime.
		
		
	 
This
  I did the free trial of Prime back in February and there was no free streaming of 
Mythbusters episodes.  It was pay only. I refused.
I had to ask myself "What do I get for $79?"  
Free shipping?  Yeah, but most (maybe all) of the Prime-eligible stuff 
also qualifies for free Super Saver shipping for non-members anyway.  Where's the membership advantage?? Granted there's a $25 minimum order total.
Instant streaming of movies and tv shows?? Technically.  They're free for a reason though. Turns out, of Prime's "Instant Video" freebies I only found 2 tv shows (+ 1 movie) worth watching.  You have to pay for the good stuff. 

 YMMV
Book borrowing? Who does that??  Kindle my ass.
Two-day shipping??  Depends.   
Not when I order just before or during the weekend. Factor in the weekend and 2-day shipping stretches to 4-day shipping.  Frankly, occasions where I can't wait 7 days to receive my purchases are few and far between. 
I also asked myself:
Am I buying things I ordinarily wouldn't?  Sometimes.  Got to recoup that membership fee, right? 
I probably average 3 orders/month. After adding all the above together I realized  tangible Prime benefits were negligible and personal profit was zero.
So I cancelled  once the trial period expired.