We've got a yellow 15L one from Rays that's about 2 yrs old now, and still going strong. As the other have said, when it's completely full it will hold it's shape if sat on the ground. I find that if you start taking water out of it though, the top quickly dips down and the water spills out. Likewise if you heat the water and the plastic goes soft and warm.
We use it for showering with - I've made a hanger that slots into the bullbar, and the bucket is suspended from that by it's handle. Drop the hose from the pump in, and away you go. Works a treat. You can use it to get the water from any nearby source, carry it back, then drop it on and shower away. Means you can't use too much water if you get carried away in the shower.
If you want one to transport water, they're great. If you expect to be able to store water in it on the ground, better to look elsewhere.
Good luck!
Matto