We've been there in many a downpour, and had fun, anyway. Put your poncho or rain jacket on AS SOON AS it starts to rain - don't delay, because otherwise you'll be soaked before you realize it. Also, be prepared for the fact that if you're wet already, the instant you head into a large attraction or gift shop - especially at EPCOT - you're going to FREEZE in the air conditioning! Bring along a sweatshirt or sweater to wear under the poncho.
And, as someone else already pointed out - stay out of puddles! Wet feet are no fun. (We always Scotchguard our canvas sneakers before our trips, and you'd be surprised how well it works. Carrying dry socks is worth it, too.)
You can't do anything about the weather, so you might as well be prepared for it and have fun anyway!:lol: