I rode to work this morning in the rain wearing a nylon shirt and shorts. They are hanging behind my office door drying now. If it isn't too cold outside, I think it is best to just embrace the rain. Wear clothes that have a hope of drying before the return trip.

I vote with the folks who are mentioning ponchos/capes. I bought one from Oregon's Center for Appropriate Transport and have been using it successfully for several months.

My commute is nowhere near as long as yours, but I do ride, rain or sun.

The poncho protects me from the rain coming from above, and keeps me cool(er) and less sweaty than if I enclosed myself in traditional rain gear. There's a constant flow of air from below, and the fenders on my bike keep the splash-up and road grime off. My feet will get a bit wet from puddle splash-up, but other than that I stay relatively dry except in strongly directional rain.

If you go the poncho route, make sure you have fenders on your bike, and choose one that is either bright or reflective.

I have to second the statement that breathable shells do work. Softshells, I don't know much about yet. "hard" shells aren't magic, and they won't work in all conditions, but they're better than garbage bags, in most cases. There are a million factors that change how well they work, but no breathable shell by itself will suck the sweat off of you.

First, I think it's important to remember that water has to be in vapor form before it can go through the membrane (I don't remember the exact reason, but it the size of the clusters of molecules needs to be smaller than the clusters in liquid water). Even so, these fabrics are passive and won't actively pull the vapor through. If it's humid outside, the vapor will only diffuse through the membrane until the humidity inside the shell matches the outside humidity (true osmosis) and it isn't instant. So, there will be some "drag" on the system. If you're sweating faster than will pass through the membrane, then it may condense and decrease the diffusion rate.

Wearing synthetic fabrics underneath a breathable shell does help keep your skin dry, by pulling the moisture into the fabric, where your body heat can more easily convert it to vapor. Cotton holds the moisture next to you, which can be a little nicer in a hot, breezy climate, but not inside a sealed-up armpit.

If it's humid outside the shell, it'll be humid inside. Venting is great, to dry sweat and prevent it in the first place. Some membranes are more porous than others, but none of them will pull liquids through.

If you're gonna sweat buckets somewhere humid; vent or just enjoy the shower. If you're not; try a breathable, but leave the cotton behind or that'll just "seal in the juices."