I love the way Tyvek performs, but I hate carrying anything that is not dual purpose, especially a big sheet of white plastic. I have a cheap but UL packable poncho from EMS that weighs as little as the Tyvek, but triples as my footprint, pack cover and a dining/auxiliary fly (until bed time). It was hugely useful for setting up in the rain several days in a row on the JMT in August, allowed me to use the stove outside the tent, kept my pack dry and accessible on the trail. I know, the waterproofing will wear, but its easily renewable with a little spray urethane, and the poncho was about 12 bucks.
