It will not give you an accurate FG in my experience, even with correction tables and formulas. But it should be within .008 or so. That's a lot, so I don't trust it at all. It won't give you bottle bombs or anything- but the difference between 1.009 and 1.016 is pretty big."
I have wrong readings after the boil, when the beer have a lot of hop particules (pellets), so it's only useful for monitoring the mash runoff and measuring the pre-boil gravity. Otherwise, you have to use your old school hydrometer. Any formulas I found to correct the alcohol reading for yhe refractometer were inacurates!