I'm enjoying Horizons for the most part: landing on planets, driving around the SRV, expanded commodities, more to explore, etc. I'm hopeful for the future: the features they have listed as forthcoming sound fun.
On the other hand, the release seems somewhat buggy. Between the API server going down and not coming back up and navigation routes resetting on each jump, I'm a bit frustrated. Hopefully they'll get those problems sorted out, although a week from launch it's surprising those issues haven't at least been mentioned as being worked on. Hopefully it's just a matter of the people who can fix them are on holiday. I guess we'll see in the next couple of weeks.