When the program works, it is great. When it doesn't, it's beyond annoying.
Do not, under any circumstances use the "improve document" option. It is a buggy mess. It will open a window within google docs that acts like a notepad and show you all of your mistakes. BUT it only really works on small documents, and tells you to try breaking your document into smaller parts. And if you implement changes, it removes any formatting from your document.
The key that I have found is that the program works on a certain number of errors before it becomes unresponsive. Write your document out and either ignore or disable the app. When you go back to edit, work through from the beginning and remove as many errors as possible (in the normal document, not the improve mode). Only by doing so will you be able to work through errors further along in the document. Even "errors" that aren't wrong, make sure to dismiss them. This will work for about 30 pages before even doing that stops working.
I really wish this program would just work without the little gimmicks. Hopefully they fix it. Especially since I paid for the "premium" version.