They work very well on a lot of surfaces. When used on glass the ink is
initially watery and barely visible, but once it dries I've found it is very
bright and opaque. (Though, just like Dry Erase markers if you scribble/put too
much ink down the tip will just move the ink about, erasing it.)
The service was excellent. They were out of stock and I got an email saying
they were back. I ordered them to use on a black white board and they really
weren't want I thought they would be they don't show up very well. My fault,
not the companies.