Here's my take on Fast Sync / Enhanced Sync.

[Man]

First thing first, I can't stand screen tearing, not entirely sure how people cope with it! With that said, I'm also not a fan of the input lag 'traditional' vsync tends to introduce. Hence it's no surprise that I've been experimenting with FastSync for quite awhile now. Actually two years, give or take! Here's my take on FastSync: 

1. Capping the frame rate is 'The Key'. I can't stress this enough. If you've had a horrible experience with FS then that's probably because you were running with uncapped framerate.
2. Your monitor's refresh rate is an important factor. If you've, for example, a 60Hz display then cap the frame rate at either 60, 90 or 120 (no need to go beyond that). DO NOT cap the frame rate at say 100FPS or 80FPS or worse, somewhere between 63-67FPS (as suggested by some Redditors). It'll cause severe 'frame judder' or micro stuttering. 
3. Contrary to popular beliefs, FS works 'Surprisingly' well at 'native frame rates'. When I play at 60 capped on my 60Hz monitor with FS, it feels like playing at ~40-45 without sync which makes sense because FS discard unfinished frames and only renders complete ones. If you're used to 30FPS console gaming then you likely won't mind it.
4. Frame rate consistency is another very important factor. Frame rate must be 99% stable for FS to work properly. 
5. Some games don't like FS. I'm not entirely sure why! Also, you can't use it with OpenGL titles, only DirectX. 
6. Lastly, FS is NOT a substitute for 'proper' VRR technology. But it can be very rewarding IF you do it right and understand its limitations.

Well, that's about it!

I hope it helps.