I wouldn't dream of going to WDW with out one, if you have a stroller. Even at none rainy times, a quick shower can pop up. DD hated it at 1st, then she figured out it keep her dry and would tolerate it only when raining. It really saved our many a trip there. At home you just don't go for a walk in the rain, at WDW you have no choice and there is nothing worse then putting the the kid in a soaked seat. Last Aug we kept the cover on when in rides, because it could be sunny when we went in a ride, and pouring when we come out.
Ours is a Graco one, it has side air vents we bought it at Target for 9.99. My friend had one from peg prego that was bigger and fit better we browed the 1st trip, I liked it better. One tip, if you get a new one, open it up and let it "air" for a few days before you leave. It is a good just in case thing to have with you.