I have that snugpak poncho. Very good for a poncho and the fact that it is closed at the sides makes it not flap in the wind as much as a regular poncho.

But it is not absolutely totally waterproof. It will get wet on the inside with any rain that is heavier than a drizzle. Not a nice feeling if you are only wearing a t-shirt and the wet sleeves stick to the arms. But that being said, it will keep the rain out. Only some of the moisture will seep through.