We tried it for a couple months, then dropped it. It was ok. Some really good channels. I had some disappointments with it, though:
1. Its UI wasn't very nice. I would have rather had a traditional program guide grid, like you get with cable/satelite. Being limited to seeing the guide contents of only one channel at a time was frustrating.
2. It doesn't run on the devices we need it to. I have a Vizio Smart TV (I only use it for the Amazon Prime app, though), and for everything else, I use Kodi on a Linux-based HTPC. Sling doesn't run on any of those. I had a Fire TV stick on our upstairs TV, though. So I used that. The problem was: since all our local OTA TV channels run through Kodi, as does 90% of everything else we watch, we kept forgetting it was there, and when we did remember, it was too much of a hassle to switch over to another device which had a horrible UI, to then find and run the Sling app, which had a nearly-as-horrible UI. So we rarely used Sling because of that. If they offered a real IPTV stream that could run through our TV server along-side our OTA channels, we'd pick it up again in a heartbeat. But that's never going to happen.
So my overall impression: Good attempt. Great channel selection. Great price. Bad implementation.