I know your most likely talking about mounted systems, but thought I would throw this in as I have fouind it to work pretty well.
Since we do several types of hunting, I have several rest systems that I employ. The most versitle so far has been the Stoney Point Polecat Rapid Pivot System. It has a detachable mount which is mounted to the front swivel post on your rifle which allows you to set it atop of the bi-pod legs for a really stable mount. The best part of it so far has been that you can purchase the swivel mounts seperately and use the same legs for several different rifles.
While this might not be a viable option for a dedicated long range rig, it has proven to work out really well for a couple of rifles that I do carry in the field. The legs I purchased in the initial set are 24" when retracted and will extend out to about 42" and allows me to use them no matter that the terrain I am in. The system is quick to deploy and very steady when in use.
IF your need this sort of versitility then it might be just what your looking for. Here is the press release on them,
Stoney Point Introduces Monopod and Bipod Rapid Pivot Systems
I picked mine up at Gander Mountian, but I am sure that many other dealers should carry them.