Apparently, if you have FireTV already, you can get some Sling for free. Amazon emailed me this:
For a limited time, get an exclusive, two-week free trial of Sling TV on your Fire TV and Fire TV Stick.
So I'm trying it out for a couple of weeks. I think anyone can try Sling for 7 days, so basically Amazon's doubling the trial length.
I'm not greatly impressed with the FireTV myself. It constantly has to buffer on Netflix, for one thing, while all my other devices on the same network have no issues. Picture quality is lower than what I get out of my PC, too, which is no great surprise given the much higher horsepower on the PC, but annoying nonetheless. And the Kodi installation can't seem to fast-forward through my MythTV backend's recordings properly, which again I attribute to lack of horsepower. Having to adjust the volume through my stereo sucks, too, since I want to leave it at the setting I've already lined up properly with other input devices. But it does seem to handle Sling OK.