There was a hot debate about this on another forum. If looked after and treated carefully then they are perfectly safe. I always attach my 6.35kg pub cylinder to a wall with a chain in case it accidently got knocked. If it were to fall over and the regulator got knocked off it could take off like a rocket.
Dave, I used to work in a lab on an industrial estate and one of our gas cylinders accidently got knocked over breaking off the attached regulator. It took off like a rocket along the ground and went through 2 brick walls before coming to a rest. Luckily no one was hurt. Mind you it was a large industrial size bottle of gas.