Lost the one off my 300D at some point, then did the same for the one on my 20D, and not too long after getting the 50D I lost the one off that. The biggest issue is that they slide up to remove, so when you push the body down into the cut foam of your bag, I use hard case cut and pluck foam in by backpack bag, it gets slid up and off. If you use your camera on a tripod with any regularity you need to keep removing it so that you can fit the little eyepiece cover that stops stray light coming back in through the OVF and adversely affecting the camera's lightmeter, which is located in the pentaprism/pentamirror housing. I once tried to get a replacement for the 300D, but the person on the telephone sales line couldn't figure out what I wanted, and kept trying to sell me a diopter correction lens holder for a 30D.
I have noticed that during a really long airshow weekend where I might make upwards of 3000 exposures a day having the rubber in place does cushion the eyebrow a little. So maybe I'll get one for this years airshow season. But probably not.