We went this past May on the 11th - 16th. We stayed at BCV. We own at SSR (can't remember when I made the reservations - maybe 6 months out?), but wanted to try Storm-a-long Bay and walking to Epcot. It was fabulous. It was hot enough for the boys to swim, but nice enough in the shade to sit and watch.
The crowds were very, very light. My son and I stayed on Splash Mountain for 2 straight times (could have stayed on more, but I was getting a little wet). We have very minimal wait times. We usually do the Early Magic Hours, but didn't need to this trip. We tried the Extra Magic Hours in the evening and found them very crowded (really usual!). Typhoon Lagoon had walk up lines on the water slides. One evening at Magic Kingdom, we walked onto every ride (Pirates of the Caribbean, Haunted Mansion, Winnie-the-Pooh, Peter Pan - which was up to 45 minute wait because the DVD just came out, Buzz - twice) during the Spectromagic parade.
I would say middle May was perfect!