Love them!
I love my slick writers! I've used them on photos but also on the Heidi Swapp ghost shapes. They dry very quickly. The colors aren't incredibly vibrant, but I think that'd be hard to achieve against any slick surface. They're a definite must have if you like to write on photos. I have also used them to "touch up" photos. For example, I had a photo where it got nicked in a spot. That part of the photo was black. I just took my slick writer to it and unless you look really, really closely, you can't even tell. Cool!