Hi Jeff,
I have used one for quite a few years for my car and caravan. The reason I changed to this type of lifting device was I had previous experience of using them to lift pipelines in the desert with ease. They are very versatile, and handy for leveling the van on sloping grass sites where you would struggle on soft ground with any of the conventional jacks. Once the van or car is off the ground they are also very stable but as with any jacking device never lay underneath without axle stands or extra supports etc.. However, you have to be careful when using them on your caravan. I always place the bag directly under the axle if possible, but if you have any obstructions like gas pipes or water waste pipes in the way then you can damage them using an air bag. If you place it under the chassis then you need to place a piece of 15mm ply of a similar size diameter as the bag to act as a load spreader between the bag and the chassis to prevent any pressure / weight on the van floor when the bag takes the load.
I wouldn't leave home without mine and as you say they are very light and don't take up much space. However you will find a lot of people will try and put you off them for the reasons stated above. Once you have used one and feel confident in doing so you will never look back.