I really like the bench rest bipods for target practice, but I'm a little dubious about their practical value for hunting or defense. Contrary to what others have said however, I find they work great from the prone, that is providing you don't have any vegetation in the way. When you use a bench rest bipod from a prone position you can curl your non-shooting hand around and use it as a sandbag between the buttstock and the ground giving you a very solid shooting position. I can easily shoot MOA 10 shot groups like this with my CZ452.
Unfortunately, in the field I find that there are things like plants and uneven ground that prevent them from being very useful for hunting. Generally you are just too close to the ground to get a clear shot at your target. I've used larger bipods as well, but I can't say I have been very pleased with them either. Generally I find that a sitting position with my elbows resting on my knees, my left hand grasping my right wrist and the forearm of the rifle resting in the crook of my left elbow to be nearly as stable as a taller bipod and I can assume that position and fire more quickly than I could with a bipod. It's not something you'd do in combat, but I've plugged a lot of critters that way.