pretty good, and I'd say a helpful community, too.
http://community.monogame.net/
Relatively recent games that come to mind are salt & sorcery, cryptark, stardew valley and axiom verge, which were all using XNA / Monogame, so you can say that the quality of games released is really up to par.
If you are going for 2d games it might take longer to setup basics than in engines like Unity, GameMaker etc., since you basically have to roll your own engine. But I it rewards you with a lot more control and insight about low-level workings of games.
I'd say for 2d games it's still worth to consider monogame, I've used it even for game jams.
At the same time there are a lot of useful 2d engines and a bunch of tools (GUI for example) that you can use to speed up development.
The documentation is pretty awesome, you can basically find anything if you google for "#problem xna" since XNA was used and loved so much.
For 3d i cannot recommend it, since it will take you ages until you have a useful base and even then you are very far away from the visuals of Unity, Unreal etc.
It is pretty good if you are looking to learn how game engines and games in general are setup from the ground up, but without the hassle of having to setup every damn stupid little driver thing and some basics like text rendering already set up like a pure c++ directx12 (just an example) renderer would.
It's also good for c# devs, since, unlike Unity, it's not limited to any .net or c# version.
