Commuting in the rain is pretty miserable no matter how you cut it. I used to enjoy the challenge of it but now I only do it because it's still slightly better than the bus.
Give yourself more time for your commute, I usually leave a little earlier because cornering and braking will be affected when riding in wet conditions. Also you might want to wipe down the bike and will probably need to change when you get to work. Maybe get some good gloves for riding in the rain. For really heavy rain I use a poncho, good for keeping the upper body fairly dry, but legs and feet still get wet.