We went May 5- 13 this year, and the crowds were very light --not as light as say, early Dec. or Sept. when we've gone, but definitely manageable. Most rides in most parks were walk-on or very minimal (<10 mins.) waits. Of course Fantasyland had it's usual 30-min.+ waits for Peter Pan, etc., mid-day, but as long as you utilize FP and/or EMH, you shouldn't have to wait in those lines!
As others have said, it WAS really hot, but everyone (CMs, regular May-goers) said it was highly unusual for that time of year to be 90+ -- they said low to mid-80s is more normal. We're going back next year about the same time, so I do hope it's closer to average!
Have fun!