Haven't used it for mounting, but I have used it for construction. It smells horrid and takes months to off-gas. It's also covered with a lubricant so it doesn't stick to extrusion rollers. Some types that are sold as decking can also have fungicides and/or herbicides mixed in. Stay away.
ALTroronto, thank you for the info and I will stay away from using it. I asked because our local recycling facility advertised selling it and seemed like a good idea. I myself have never used it and had no idea what it was like.