Spaceception

I've used it, for school. I wasn't too impressed by it, and I wouldn't get the paid version. I can remember distinctly an argument in a group project about who was right; me or Grammarly. The other student insisted that she was correct because "Grammarly agrees with me!" Turns out that she was wrong, and Grammarly was, too.

You'd do better to learn the grammatical rules yourself. Not in, say, English class, but by going out and reading a well-written book in a subject you enjoy. For me, it was history, but it can really be anything.

To sum up: Grammarly's OK, but it's nothing to write home about. You can get the same thing by finishing papers early and asking teachers to proof them.

Is it good for generally fixing some grammar mistakes?

I guess so, yes. But I still think it's a good idea to learn the grammar rules concurrently. You don't want to become too dependent on technology. Also, sometimes you have to break the grammar rules for effect. Knowing what they are, and how you're breaking them, will help you.