I'm not in a typically rainy climate, so generally I prefer a breathable soft shell outer pant, but I DO use rain pants for 3 situations.

1 When it's cold and raining in the spring/fall - alone or with a very thin base layer.
2. When it's snowing - obviously it's cold, with varying base layer thickness depending on temp.
3. In the dead of winter (under 10F) as an outer wind blocking layer with a thick base layer or thermal tights underneath.

in the summer I don't bother - no matter the claims of the manufacturers none of them breathe well enough to keep up with aerobic activity on a wet day. so you're wet no matter what. That's what towels are for after the ride.

Mine are from REI, cycling specific cut. They don't make the exact ones any longer but they do make something similar. They were a worthy investment.
http://www.rei.com/product/819020/no...ain-pants-mens