I have always been quite lucky with the weather, it has so far not cost me any +1's, although it has come close, we had a lightning delay of over 2 hours at SFNE that nearly cost me Goliath.
Then we had a tornado in Sandusky whilst at Cedar Point, which obviously shut the park down, I had never heard a tornado warning alarm before and although quite unnerving it was at the same time quite exhilarating in a "I feel alive" kind of way. Although it never cost me any coasters as it was on day 2 of our trip and we had already rode everything, even so, the weather cleared up, the sun came out and we rode Dragster? Just a vary surreal experience.
I brought the local paper next day and have kept it, it done some minor damage but nobody was hurt I don't think.
Like Ben said, bad weather is part of the hobby and it is something that can hamper your trip whenever it chooses, I always make sure I have 2 days at larger parks to ensure I can get everything done should weather play a part.
However having said that, we will playing with fire next month, I have 1 day at Carowinds, BGW, Kings Dominion, SFA and 2 at Dollywood, so 300ft+ rides in late October could be asking for trouble, we will see?