I ride in a trenchcoat sometimes. It's OK. If you don't button it all the way to your knees, your thighs get wet; if you do button it to your knees, it's hard to pedal. It's definitely no substitute for rain pants.
My wife wears a rainwrap (i.e., rainskirt) from
Georgia in Dublin and loves it. I've been trying to convince myself that my masculinity can handle it, 'cause it really seems to be a great solution -- more protection than a trenchcoat, less annoying to take on and off than rainpants.