So what do you wear if you'll be out all day in the rain? There are a couple options:
1. fancy-pants gore-tex rain gear (for example, this at a fairly standard price)
2. el cheapo rubber rain suit you pick up at the hardware store (like this one)
3. poly-coated tyvek from the health and safety pile
4. Bah! I'm way too cool for a rain suit!
I think I've tried all of these at one time or another. The disposable poly-coated tyvek gets too expensive over the long term, the super cheap rain gear from the hardware store will tear if you look at it the wrong way and will be way too hot, and going without anything all day is pure misery.
So I've sucked it up and gotten a really expensive, semi-breathable rain suit. It makes me cringe when I plunge through thickets of thorn bushes, but so far, my current set has held up to about 2 years of steady abuse and rain - not a bad investment.