We have a Roku and do not have any experience with the others. Like Circuits, we mostly use the Roku to stream stuff on Netflix and Amazon Prime. I think Sling TV would be a good complement as well. I also use the Roku with my Plex server, but recently got a warning from Comcast, so I'm not sure how often I'll use that in the future.
The Roku has more and more channels all the time, but most of them are like watching crappy cable channels in the middle of the night, on youtube. Except for Netflix, Amazon, Plex, and Pandora, most of the other channels get watched at 3:00 am when I've been drinking just to see what cool stuff I can find. Redbull channel is pretty cool, but I'm not really into sports so don't take time to watch it. There are some cool gun channels, a pretty decent Homesteading channel, lots of cooking channels, a handful of decent hunting channels, and quite a few good workout channels, BUT it's all stuff that you could most likely find on Youtube, or is exactly the same content on that same providers Youtube channel.

I think with SlingTV, it'd come pretty close to replacing cable.