Killer Instinct is the game that Microsoft has chosen to provide free to every Xbox One buyer, with a single character. The other's you'll have to pay for with DLC packs. While no one is forcing you to pay and certainly no one is forcing to play, chances are one of the first experiences anyone will have with the Xbox One is with Killer Instinct, so it better be good.
With that in mind, here's what the professionals have been saying about the launch title:
Compared with the relatively poor reviews of Dead Rising 3 's visuals, Killer Instinct does well in this respect. It has pretty particles and pretty lighting and the roster of available characters all look great. However, that roster is woefully small for a modern fighting game. It might have been acceptable back in the days of the NES to have just six fighters to pick from, but in a next-gen title, it feels decidedly old school; and not in a good way.
The core of the game though, the mechanics, has been near universally praised. It has a great combo system and multiple viable ways to win a fight. Similarly, there's a fantastic built in tutorial system that works to teach you not only about the game itself and how to play as each of the characters, but general fighting game meta tactics that should work across other titles too. Clearly a lot of work has been put into making the game accessible for beginners as well as fighter pros.
This works out great for the multiplayer mode, which allows near-infinite competition with like minded fighter fans around the world, but with no single player, you're forced to go up against real competition if you want to do anything more than train.
The lack of characters will definitely begin to show after a while playing though. With a single (possibly rotating) free character and only five others at launch, you're going to face the same ones over and over without much difficulty. More characters are planned in the future, but the game smacks of being somewhat unfinished - and suffering a little from its pedigree, since Killer Instinct's previous instalments have never had massive rosters.
All in all a good game with a solid fighting core, but it's a little less varied than real fight fans will be used to.