OK, yes it does provide 3x magnification of the rear viewfinder, but for stills photography, it's not good enough quality to fine tune focussing etc. However, if you use your camera for video, this is 100% essential.
The base plate is glued to the camera it can be removed, but not continuously. I thought long and hard about this and whether to buy the eyepiece adapter to attach it to. BUT, as it's a superb protector for the LCD viewer, I decided to go ahead and fix it to the body of the camera as added protection. When the time comes to sell my 5D MKII, I'll sell this loupe with it and leave the LCD protector in place; should be an advantage when selling :