Ditto on not doing it in the rain. That's what the car's for.
Rain is EXTRA reason to use the bike. I usually drive once every couple of weeks, just to keep the car from rotting out, but if it's raining on the day I'd planned to drive, I'll postpone it so I can ride in the rain. I suppose if it rained all the time around here I wouldn't enjoy it as much.
Ditto the comment that 'rain is what fenders are for'. That and rain gear.