They are very water resistant, maybe even "proof", but that just means that you heat up underneath and end up wet anyway.
If you fold them and wear them as a belt, just in case - it gets icky very quickly. So i only use them if it is raining when i'm leaving home. For the chance of rain - i rather get somewhat wet.
Lately i've discovered that you could use them without closing the velcros at the knee, thus keeping yourself much more ventilated while not compromising coverage. But i don't know how that would work on a faster ride.
They don't cost much, so i recommend them. Mostly because in cold rain (unlike what we have here) they may work great, because you don't sweat as much, and your body cools off alot faster.
I used them a couple of times on my road bike, when i went for a ride on a really rainy and cold day - and stayed fairly dry, mostly because i was just rolling along, not putting too much effort into it and not heating up.