I used up my free-month trial. It's freaking awesome! However, all it did was made me buy a few more things that I would have wanted. Well, I needed them but I could have waited a few months. The 2-day free shipping is the best! I didn't really care for its movie streaming since we already have/use Netflix and Hulu+.
If you're a college student, you can get 6 months of free Amazon Prime without the streaming service. After that, you pay half of what it costs for the year and they give you the streaming service as well.
If you want to work the system, you can split the cost of Prime with upto 3 or 4 buddies (forget the limit) and add them to your account so that they'll enjoy 2-day free shipping but they won't get the streaming service.