I have them. They are not a replacement for a winch. I carry Max Trax, a Wyeth-Scott Company "More Power Puller" with extra synthetic rope, a hi lift, shovel, tire chains, air jack....you know the drill and have never needed anything other than my Bubba Rope and buddy in his ride.
There's no possible way that could replace a winch. It will make certain types of recoveries possible, but it won't help you up that waterfall or work to pull a buddy's FJ out of a tree that he slid sideways into.
I recommend you take a look at past issues of Overland Journal. They did industry review on sand ladders, bridging ladders, maxtrax, etc. Can't remember which came out on top, but I don't think it was Maxtrax.
Definitely, not a replacement for a winch, but its probably better than nothing.