I use a watermark on all my photography on an art website. I've had a lot of mixed messages for them- some people incredibly abusive that my work has a watermark, and some people like the creativity I use to add them.
I used to use one that spanned a chunk of the photo, and I got comments on how ugly it was, but at the same time, I could see it being posted over the internet WITH my watermark, so it was obvious where people had stolen it from or tried to claim as their own.
Now, I incorporate the watermark into the image, following lines or curves like this:
http://xerces.deviantart.com/art/Drive-185384391http://xerces.deviantart.com/art/Crystalline-Beauty-188578759That way, it's still there, still visible, but not obtrusive