I wouldn't say "ruined" my vacation, but I wasn't too happy about it.
Have gone August 2009 and August 2008. We knew about thunderstorms, rain, etc. which was fine. There were torrential downpours during these trips, but most of the time we were prepared and "weathered the storm" so to speak without any issues. Actually the one night in August 2009 there were over, I believe 10,000 lightening strikes that night due to a huge storm that had rolled in.

But we were back in our room before the worst of the storm hit, so we just stayed indoors.
We went in April 2010 and everyone raved about how wonderful the weather would be in April; it's not too hot, it's beautiful weather...blah, blah, blah. This was actually the first time I planned a non-commando trip so we could enjoy lounging around the pool a few days, etc. Needless to say, the sun didn't come out until the day before we left and I was miserable. It was cloudy, cool, rainy the whole week we were there. I guess the influx of cheerleaders and the mass of teenagers at the parks didn't help either. And FWIW, I'm one of those sun junkies. If the sun is out, I'm in a great mood. If it's cloudy or raining, I'm depressed and upset.
So what did I learn from this latest trip in April? I WILL NOT go back to Disney in April EVER. As I said, it wasn't just the weather, but I hated being there with the cheerleaders and the class trips, senior trips, proms, etc. just weren't for me. I love the end of August when it's hot, humid and EMPTY. Guess I'm just not a people person.
Just thought I'd add: I go to Florida to be in the sun, which I know isn't guaranteed, but it was actually nicer at home in NJ that week than in Florida.