I love my 600 dearly, but if I were thinking of buying one right now I'd wait for the next generation. The weight of the new 600 is considerably less than the current version, and for someone like me (who takes the lens hiking and kayaking) the weight becomes a real drain. My shoulders usually have bruises on them from slinging the lens/tripod over my shoulder while hiking.
My use of a monopod is dependent on how far I'm hiking. I use my 600 almost exclusively for birding, and I'm an equally avid birder, so I'm often hoping to see the bird. That often involves looking through my spotting scope. It's very nice to have the tripod where I can leave the camera by itself and not have to try to find a safe place to set it down, especially if I'm on the beach looking at gulls up close or distant birds on Lake Michigan. That said, I've usually thought the monopod would be equally effective in most of the bird photography I would do if that were the only thing I were attempting to do.
Where I've found the tripod to be invaluable is using live view on stationary birds. I've found that I don't always get absolutely precise focus when trusting the AF by itself and using live view allows for that.
Here are a couple recent shots with my 600 and 7D.
My 600 is a UN date code. I had a UW 500IS I had previously that I traded plus a little cash for the 600. In my experience, my 500 was just a tad sharper at f4 than the 600, otherwise I've found them to be remarkably similar in performance.
