Here's an older thread on Viking composite propane cylinders...I own them and I love them. Very light and well made. Hexagon Ragasco (the Norway based parent company) products were never part of the DOT recall on composite cylinders. The cylinders weigh substantially less than even aluminum cylinders, and are about half the weight of a steel cylinder (a 17lb Viking cylinder weighs 10 lbs; a 20lb Manchester Tank aluminum cylinder weighs 17 lbs). I'd recommend them any day...
I converted from dual 30lb steel cylinders to dual 20lb composite cylinders and saved over 60lbs of weight from well behind the rear axle of my truck. It's one of the cheapest ways to drop that much weight that I can think of.