A beanbag is a very useful camera support and can be one of the best ways of supporting a long telephoto lens in some situations. I regularly use a beanbag (or two) instead of a tripod for supporting my 600mm lens when shooting landscapes. It is my preferred support when shooting wildflowers and fungi and when photographing wildlife from a vehicle window or roof.
I use double beanbags from Wildlife Watching Supplies. It’s vital to choose the correct filling for any beanbag. Never choose a filling because of its lightweight properties – stability is important and lightweight fillings such as polystyrene balls will not provide a stable support. Hard polypropylene granules create a stable support and are also waterproof. However, I prefer to use a wild birdseed mix inside a plastic bag placed inside the beanbag. This provides a really stable support and can occasionally be useful in helping to attract wildlife a little closer! The only downside is that it needs to be changed regularly if you do intend to use the filling as a food source for wildlife.
When travelling by air my beanbags are packed empty so save space and weight. When I reach my location I buy a bag of cat litter as a very cheap but effective temporary filling while I’m away. When travelling abroad on a wildlife photography shoot I also use a beanbag as a substitute to a ball and socket head if I want to shoot the occasional landscape from a tripod. I simply place it on top of my fluid head (which I use for long telephoto lenses) eliminating the need to carry an extra tripod or to switch tripod heads.