We went may 5 - 13 this year...and what others are posting has been our experience as well. We have visited during most seasons at one time or another, and I would say that May weather was hotter than I would like it, but certainly able to deal with it. Not too bad.
Same with crowds--the parks "felt" crowded, but we never waited more than 15 minutes for anything. Disney is the business of moving people, don't forget, and they did a great job with this. We stayed at SSR, which was at capacity, and did not feel crowded there at all. Don't kow how they managed that! lol
However, I will likely not go in May again. We have been too spoiled by Septembers trips--everything is a walk-on and you get the characters to yourselves, though it is really hot. And Jan/Feb, where again, everything is a walk-on, though sometimes too cold to swim (though we have only had that be the case once).
Happy planning!