I have a RedOctane dense foam version in my closet that I was given for Christmas one year, and it worked really well actually. The only drawback I saw in it was that it still slid around on the carpet more than I wanted. My Cobalt is really amazing, but I basically bought it 3 months before I stopped playing DDR on the pad. It just sits next to my bookshelf now, looking somewhat dusty. I got the arcade panel mod for it too but I disliked how the buttons responded after that, so I removed the mod. Otherwise, it was extremely functional and hardly ever shifted but maybe 1-3 inches.
The cobalt also weighs a ton. Close to 50 lbs if I remember correctly. It is also not very transportable.