I have them on all my shotguns... Easy to install, remove with no marks. When empty rip off the shell holder grab another and boom, you have a full she'll holder again. I bought mine from SJ Hardware along with the small zipper pouch which holds two full shell holders inside and loops on the exterior. All in all pretty simple and practical.
http://sjhardware.com/store/modules.php?name=catalog&file=index&cPath=107_125&osCsid=1c7c774d1f52f4072218a1cc4073f82f