My Bernina extension table is not flat for some ridiculous reason. It is thicker (higher on top) on the right side than the left & curves downward toward the edges, but I have no problem using my Slider. My Slider has a micro-crease in it where the bobbin viewing window is. It doesn't affect the functionality of the Slider.
You do not want to use it with your feed dogs up or it will get chewed to bits. I've heard too many stories about that happening, so I try to be very careful to always remember. You can do straight line stitching with a darning/FMQ foot and the feed dogs down, but if you attach a walking foot, you will want to remove your Slider or it will get chewed up top and bottom.
EDIT: If you really want to use your Slider with the feed dogs up (some people FMQ with them up), I know 2 girls in my guild that used scissors to enlarge the needle hole in their Slider. They claim that works. I have no idea, but at least the Slider wouldn't get chewed up.
Last edited by Bree123; 12-11-2015 at 08:30 PM.