They are still perfectly practical lamps, but have a few snags. On most of them, the front light is quite scattered so that although giving a lot of illumination, it is not focussed very well. That is not true of some of the Powell and Hanmer ones. The rear light is usually supplied with gas from the front lamp and the long feed tube seems to give trouble. Unless you keep stopping to look, you don't know it has gone out. To put things in perspective, in the early twenties, an acetylene motorcycle lamp worked a lot better than an electric one. By the late twenties, that was not true. Until comparitively recently, an acetylene cycle lamp cast a better light than a battery lamp. By 'recently', I mean fifty years ago.
I don't think they are particularly heavy. A lot of the weight is in the water you fill it with; a full bladder must weigh a lot more, which brings me to one of the legends of acetylene. Old- timers always claimed that when they ran out of water on a ride, they would pee into top of the lamp. As the filler hole is about half an inch across, they must have urinated a lot more accurately than I could even when I was young!
If you have a lamp, give it a try. If you are worried about the rear one, you can always dangle a modern flashing light from your helmet.
Stuart Tallack in West Sussex