Quite good for English. Also one of the best checkers for Russian. There are some issues, but they all can be reported on their github and they do fix those.
Hab das Tool zum schnell überprüfen der Rechtschreibung installiert und funktioniert top, da man ja doch oft vergisst auf Groß- Kleinschreibung zu achten.
I have been using it mainly for European Portuguese, and it works very very well. It is not obstructive, the tips are quite reasonable and it is very effective.
