We have an amazon.com credit card and we love it. We don't do a lot of flying, so we weren't interested in a frequent flyer card, but you can buy pretty much anything on amazon.com... last year I saved all the gift certificates and did almost all my Christmas shopping on Amazon, and barely spent any "real" money on Christmas.
We use it for everything - groceries, gas, etc. so we get a lot of rewards, but never carry a balance... it's just easier than carrying cash, and easier to track and make sure we're sticking to our budget. You need to make sure you're able to only spend what you have the cash for before deciding to use a CC this way though.