Ponchos have been recommended in the Camino forum since they give the protection fro the rain, but also let air flow underneath so the heat your body produces can get out.
I have always preferred using a rain jacket and trousers to a poncho. In Late August we set off from Roncesvalles in torrential rain. The rain ran down my back, between jacket and pack cover, it poured down my trouser legs into my boots and I ended up drenched. We bought a ‘raincoat-poncho’ which is a combination of poncho and raincoat. It is a long rain coat with sealed seams, ventilation flaps in the front which zips up and has velcro stays; long sleeves with velcro to tighten around wrists, a snug hood with a peak and an adjustable ‘hunchback’ at the back to accommodate a backpack. It really was the answer in the rain. Better than a regular poncho which can blow up in the wind and which wide open sleeves don’t keep your long sleeves dry.
More in the Camino de Santiago forum.