If you already have Amazon Prime, a Firestick is a good starting point even if you have a smart TV. And if you don't have a smart TV, it turns your dumb TV smarter.
Firestick, by itself, is not a service -- it's more of a portal to other services. For example, you can access Netflix, Hulu, etc through the Firestick as well as all Amazon Prime digital content.
In addition, you can download the apps for your favorite cable channels and access them through the Firestick. Your experience will vary by app -- some give you full access to their content on a delayed basis but require you to watch commercials to access it. Others limit the content or charge a monthly fee for full access.
We have a Firestick on our primary TV and use it access:
* Amazon Prime Digital Content -- both free programs covered by our Prime subscription as well as rentals thru Amazon that will play over the Firestick-enable TV
* Netflix (monthly fee applies)
* HBO GO (we're still with our cable provider & have HBO as part of the package, but HBO offers a monthly subscription option for cord cutters)
Because we're not yet cord-cutters ourselves, I don't really have any apps from the different cable channels.
I have no experience with Hulu, Sling, Roku, etc -- but if you already have Amazon Prime, pick up a Firestick and test drive it. Amazon has refurbished ones for $35- or new for $47-.
If you don't have an Amazon Prime account, please get one immediately. Jeff Bezos is divorcing and needs the cash.