Wonderful time to go. We went the first week this past May. MK and MGM were empty. No lines on most rides. Max wait was 20 minutes on the most popular rides. Epcot had crowds because of the flower show. AK was very crowded when we went both times (a Tuesday & Friday).
The weather was perfect. Mid 80s each day. Hot in the sun. So swimming and water parks are good. It rained twice for 15 minutes the whole week (at be it, a complete down pour for 15 minutes).
Mother's Day weekend is nice. Epcot has a really nice lunch on the Sunday and mothers are given flowers as the enter each of the parks.
There is a concert series at Epcot and the flower show is really nice. Our kids loved the butterfly garden and Pluto's playground (both there only for the show). But as I said, the park was very crowded. Make use of fastpass on the popular rides.
Check out which days AK has morning magic hours. Get there for the 8 AM openning and hit the popular attractions first, eat some breakfast, then do the shows, other rides, and Rafiki's Planet Watch.