Be afraid... be very afraid.

If it is warm rain I just get wet but for the cold, never ending, bone chilling, soul sucking rain you get in Oregon during the winter you need to gear up.

Waterproof shell jacket and pants, mebbe a little wool for a mid layer, waterproof gloves, and shoes that are either waterproof or covers to make them so... my spd shoes are sealed under the sole to prevent water from seeping in and the shoe covers keep water out.

And if you are gonna run fenders go all the way... they protect you from stuff that may not be water and do a lot to protect your bike and drive train from sucking up water and grit.

PS... make sure your stuff is also waterproof by using a waterproof bag or zip lock baggies as you do not want your stash to get wet..