It's great. I use a 55" Samsung smart TV and the HD quality is great. The only platform that YTTV isn't available on (currently) is the Amazon Fire Stick or Fire TV, but i'm sure that will change in the future. You can download the YTTV app on Roku, Apple, etc.
Here's a link that I used for researching when I was trying to decide between Sling vs. YTTV. It's a little outdated, as the price quote is incorrect. It's now $50 a month but still worth it, IMO.
YouTube TV vs Sling: Which should you use?