Halo Wars 2 is not so good that it will influence the direction of RTS games for decades to come, but its also not bad enough to be something you should actively avoid. It sits in that strange middle-ground, and if you're like me and you're not sure whether its worth your time and money, you'll be glad to hear that Halo Wars 2 now a free demo available.
You can find it by heading over to the Microsoft website, but before you even begin the download its probably worth mentioning that the demo is ~17GB in size. As far as content is concerned, the demo features the opening mission from the campaign, as well as a good look at the Blitz Firefight game mode where you'll get to "use decks of cards to summon vehicles and troops to fight against waves of AI-controlled enemies."
If you're interested in a better look at what exactly Halo Wars 2's multiplayer has to offer, you'll find find one of the pre-release preview videos right below. Its a bit overly dramatic, but it should give you a good idea of what exactly you're getting into without having to go download 17GB of data beforehand. Have a look: