I am a heavy Amazon Prime user, placing probably about 100 orders per year. (I also order groceries online from a local place. It costs $5 per delivery but many of the same benefits apply.)
* It's super-handy if you don't have a car. No lugging heavy stuff around on a bus or bike!
* It's also handy if you're the kind of person who hates spending time in stores, particularly big box stores. To me, navigating huge parking lots and wandering down endless aisles to find a single, particular object is a hellish, life-sapping experience. Amazon obviates the need for that. And it's one-stop shop. You can quickly place one order for a miscellaneous but highly-specific variety of things that it would take all day to find in the physical world.
* I find it much better to shop for things from the comfort of my home -- can quickly price-check with other sites, read reviews, research comparable products, etc.
* If you have Prime and use an Amazon credit card, you are refunded 5% on Amazon purchases to use on future orders.
* The free movies are a nice add-on. The selection isn't large, but they occasionally have really great arthouse or other quality films.
All in all, it is well worth it.
The downsides and worries include environmental issues, labor issues, and the fear that Amazon could someday become a monopoly and drive prices way up. I do patronize our neighborhood mom & pop hardware store, buy used furniture and other goods off Craigslist, and so on. But for many kinds of products, Amazon has been the best option, so I use them despite my misgivings. They do what they do really well. And I don't think replacing Amazon with big-box store purchases would be much better.