I had never even seen one of these "Eye Cup" attachments until the photography studio I work for bought a spare camera off of eBay and it arrived with an eye cup installed. After using it a few times I knew I wanted one for my Canon and ordered this. It arrived extremely quickly, is priced right, and is a pretty decent quality. The first time I took my camera body out of my bag after storing it with the eye cup, I was worried that the eye cup's rubber shape had warped due to being smashed in the bag. If that happens to you, don't fret -- the rubber is pretty malleable and will return to its shape over time. If the temperature is cold, it's going to take a lot longer, though, obviously.
I wear glasses and have found that using the eye cup is a lot more comfortable for me when shooting through a viewfinder. I imagine that without glasses, using an eye cup would be even MORE comfortable.
Bottom line: this is a love it or hate it accessory, certainly, but I think most people would find that it's a nice addition to any camera body. It's very easily removable and replaced with the stock viewfinder attachment in case you find a need to remove it. And lastly, on a superficial note, I think that an eye cup makes any camera look more robust and professional.