I have a campmor 5x9 extended poncho tarp and its decent, but I would not want to be in really nasty weather in it.
Besides if it blows away in a storm and you dont have any spare rain gear etc you are screwed.
I thought the tieouts looked a bit weak on mine so I added some reinforcing.
Weighs 9oz.
You need a good well drained campsite, a bathtub GC and a bivy of some sort.
They now make a driducks emergency poncho that weighs 2.8 oz and is cheap so you can carry that if you have to set up in the rain, use it for extra protection etc.
Also carry your cordage in a bag with small biners so you can hook onto the poncho tarp.
All additional stuff.
Actually I think its better to carry a rain jacket or a parcho and a cat cut cuben tarp and keep everything separate, but if you are trying to do the minimalist sub 5# thing a poncho tarp will get you there.