The problem I have with most shoulder mounts is that they are not designed to balance once the camera + lens + monitor + matte-box is mounted to the mount, this means that your arms can get very tired/shaky within 1-2 hours of filming.
The only mount I've really liked so far ( the one that I will probably buy later this Spring ) is the...
It's not cheap, but it's very configurable, and it can be set to an almost perfect balance with no weight on your arms. You can film all day with no arm fatigue. It comes with a lifetime warranty, and will keep it's value if you ever decide to sell it.
I owned the RedRock Micro shoulder mount for 6 months, and then sold it because my arms could not support the weight of a fully rigged camera for more than 30 minutes of shooting. The newer version of this shoulder mount finally has counterbalance weights, but I've never had a chance to play with it.