The first, and only time I have ridden Millennium Force, it started raining right before we boarded, and continued thoughout the ride. While I loved the ride, and will be going back this week to ride it again, I can say with authority that those raindrops HURT!!! Well, at least when they're smacking you in the face at that speed...
By the same token, it's much more pleasant than the bugs smacking you in the face in the front seat of Magnum...