I am a big fan ponchos. Serving 24 years in Airborne Infantry and Special Forces, I have spent literally months, or even years, living in poncho hooches. Their versatility as rain gear, shelter, and emergency litter make them a valuable piece of gear.

As for criteria, the usual attributes of size and weight are a factor, and the material they are made of affects the weight most of all. The military issue ones are not the lightest, but are durable. I currently carry a Sea to Summit soil-nylon poncho. The dimensions are greater and it is light and packs small, but it is not cheap.

As for making a shelter with a poncho, there are so many ways. With a single poncho, the lower to the ground, the drier and warmer you will be. There is that inconvenience of getting in and out though. I used to carry 2 ponchos, as the military ones snap together and can make a larger shelter, drier and easier to get into along with your gear. You do need to tie up the hoods to make sure they do not allow water in. It helps if you then guy them out so they do not form a depression in your shelter that catches water. The ends will be open, but can be blocked with gear or natural material if that is available.

Ponchos are used much like tarps. Tarps can't be worn as rain gear, but they are essentially the same in making shelters. Any guide for making tarp shelters will be useful for poncho shelters.

Some pieces of kit that I would never have considered going into the woods without when I was in the Army:

Poncho

Canteen Cup

Canteen

Knife/tool

550 cord