I currently have a temporary background, and will be making a more permantent one. For some reason, I have heard no mention of the material in these forums....Fabric.
I dont like how paper or plastic backgrounds attach to the tank, and the edges eventually fray or peel away. Painting is too permantent. I love the rock backgrounds, but between my incompetant artistic ability, and the loss of tank space, they are out for me.
I think the fabric looks a little better, as its a flatter color, and the texture gives it just a little extra that plastic or paint cant do, in addition, if it gets wet, it wont get ruined. Once I figure out how to permantenty attach it (im leaning toward mounting clips so that I can change it when I get bored of it) it should start to look pretty good.
Although, to be fair, I think the goal of any background is not to be seen. In my opinion, they are used to just block the dead spaces where you can see through the tank, so you are not distracted by something behind the tank. The real goal is to get my plants to grow in thick enough to block most of the background.