When it is working it is a fun game.
If I had a complaint about the game unrelated to technical issues, it would be that there is still too much structure. Compared to the first two games, 3 is a little looser when it comes to the overall imposed structure the game puts on you. Events are now more user controlled like in 2, and with the shift of the tone of the game from "participant" to "boss" certain aspects feel more user-derived rather than simply you doing what the game tells you to do. But the options the game presents you with are all "on rails"... things unlock when the game wants them to unlock, there is no "driving around on your own and discovering new events". You CAN drive around and treasure hunt the billboards and post card places, but all the events, stunts, barn finds and showcases must first be triggered by the game.
The only other minor gripe I would have is that unlike say Gran Turismo, Horizon is much more focused on supercars and exotic / rare cars. There are like fifty different Ferraris in the game but only like a handful of "regular" cars (that aren't some kind of rare collectible version). And since 3 takes place in Australia Holden cars are kind of over-represented. It is more like a big case of Hotwheels cars than it is a representation of the driving world at large. So if you like racing LaFerraris and Bugattis then you are in luck... if you like racing Nissan Stanzas and Ford Escorts you are SOL.
But if all you want to do is tool around on your own you can do that... it just gets kind of boring after a while and you go right back to the structured events.