We have gone the last four years in May at various times (first, second, third week). We have never gone around Memorial Day so the following is pre last week of May.
The weather varies. But it is hot at least upper 70's if not low to mid 80's but nothing like August was when we went about 10 years ago. There are the occasional freak rainstorm but for the most part the weather is sunny and clear. It can quickly get cold at night so bring a long sleeved shirt or sweater to pull on over your shirt if you stay for Fireworks, Illuminations, Fantasmic or just stay out late at Downtown Disney.
As far as the waits: most rides and attractions have wait times around 20 minutes or significantly less like 5 or 10 or 15 minutes if it is not a popular ride or it is a downtime. There were many occasions that rides like Buzz Lightyear, Small World and Mission Space had a wait time of 5 minutes. But then there are the big ticket rides that always have a long line. I never passed Expedition Everest without seeing a wait of at least 40 minutes. And Soarin' was always at least an hour wait.
Overall the weather is pleasant and the crowd sare lighter because about half the public schools in the country run through June. On the other hand there always seem to be numerous groups of 12-15 year olds wandering around in matching shirts (bands, choirs, dance groups?) Groups of tweens and teens left to run around Disney can get a little annoying but overall May is a great time to go.
FYI about ADR's: even though May has lighter crowds you will still want to make reservations for breakfast, lunch or dinner if you have your heart set on a specific restaurant. For lunch or dinner you may not have to make reservations now but make them early on when you get into the park. If you wait until it's time to eat dinner you'll find that there is no table service place left.
