In general, here in the land of cold(ish) rain, I've found it best just to wear shorts and t-shirt and soak right through, then change at work. I don't bust out the long pants and burley bike raincoat until it's too cold to be soaking in rain (Nov through Feb-ish around here). Then even if I do sweat from the raincoat, at least the sweat gets warmed up metabolically!

But that burley raincoat is well-vented; I've never quite been able to verify if the wet spots on my shirt at the end of my ride are from sweat or from rain leaking through some of the openings, that I generally leave cranked all the way open except on the very coldest days.