As someone who grew up in the SE Alaskan rainforest & who currently lives near Seattle, my opinion is that it's not about staying dry so much as it is about staying warm. Aside from getting trenchfoot, having your body get wet is basically an emotional discomfort. Most of the danger from rain is that it makes you cold and decreases the insulative properties of your clothes.

In the short run, most raingear or ponchos will keep you warm & dry if the conditions are right. In the long run, though, you will get soaked from water or sweat no matter what you wear. The best you can do is find the raingear/poncho that will keep you comfortable under most conditions in your climate at your typical activity level.

From what I've seen, most Alaskans swear by Helly Hansen while most Washingtonians prefer Marmot. Personally I find Martmot's Precip to have too many bells & whistles, and for the last 6 years I've been using Cabela's ultralight raingear.