We typically visit in September. Yes, it's hot and humid, but I'd NEVER describe it as rainy. Maybe an afternoon shower, but that brings some relief from the humidity.
We went this past May and comparing the weather to that of September was a major objective for me (the trip planning, spreadsheet making geek of the family). I'd say the actual temperature was as hot as September, but it didn't feel as humid to me.
The crowd seemed larger in May. Especially when we left WDW and visited US/IOA. There were lots of school groups at those two parks. We've never had much of a wait at these parks, but during this trip we actually purchased express passes for front-of-the-line access. It was an extra expense we hadn't anticipated, but without it, we would have spent most of the day standing in line. Though unexpected, let me say, it was an expense well worth the cost involved.
Aside from the weather & crowds, another consideration for us when comparing May and September would be the MK after-hours parties. We did the Pirates & Princess Party in May and had an absolutely fabulous time. We also had a fabulous time at
MNSSHP in September 2005. Either of these are lots of fun and, I expect in the future, we'll choose to visit only during a time offering one of these parties (or
MVMCP). These parties offer a chance to experience MK when the temperature is more pleasant and the crowd is much lighter.