Far better than whatever the critics say.
First of all, the story is really solid, albeit confusing at times. You start off knowing that something happened to humanity a long time ago, and the world isn't faring too well right now. The main protagonist's daughter is sick with a deadly disease, and the game is basically about the father's journey to find a cure, as well as much, much more. I'd say more, but I don't want to spoil the story.
One thing that's cool about the story, as with most RPGs, is that you can have a New Game+, which basically means you start over and play the game with generally more stuff to do. That applies to Nier, except there are FOUR different endings you can view, which means three more New Game+s. Each ending is really different than one another, which gives one motivation to play through the whole thing.
Anyway, enough about the story. The graphics aren't Final Fantasy, but then, what is? That's not to say the graphics aren't beautiful, however - they most certainly are.
The gameplay is pretty entertaining, for the most part. Combat is very satisfying, with blood spraying every which way. There are thirty-ish weapons to collect, all which have their own stats and look different. They are also upgradable, though upgrading weapons can be a cumbersome process. As you progress through the game, you will also gain 8 uniquely different spells, all that have their own uses like summoning lances out of the ground, slamming a giant fist upon your foes, or creating a shadow of yourself to almost instantaneously attack your enemy. So, the combat, once again, is pleasantly satisfying.
You can also spend your time fishing, which at first is extremely hard. Though, sooner or later, you'll either quit at fishing or find it extremely easy. You also have a little garden where you can grow fruits, vegetables, and flowers to use for side-quests or sell.
Ah, side-quests. The side-quests are easily the most controversial part of the gameplay and tend to act as a scapegoat for critics' rage. It is true, yes, that quests will have you running back and forth have the time. It can be repetitive, yet it's still not quite as boring as critics make it out to be. There is also often unique dialog you hear during quests, which can make it more bearable. Oh, and did I mention that side-quests are COMPLETELY optional? That is, unless you want to unlock the 3rd and 4th endings, which will require you to complete only a fraction the game's numerous side-quests. And for those of you that love extra gameplay? Well, side-quests are your answer.
One of the places where Nier shines the most, however, is the soundtrack. It is rare to ever hear a soundtrack so amazing. The soundtrack perfectly sets the mood, whether that mood is melancholy, happiness, joy, or just about any emotion that you'd expect in a story such as Nier's. In fact, I found this music so great that I even found myself buying its soundtrack.
So: the bottom line, in my opinion, is that Nier is a gem among the numerous RPGs out there, and an underrated one at that. It does most things quite well, and these things make up for anything one may find that Nier lacks. You really must try this game if you like RPGs.