Hi -
We went last year over Memorial Day weekend and found it to be a fantastic experience. It was much better weather than what I previously experienced in July. It was warm but not hot, and it didn't rain every day. The crowds were less than what I had expected for a holiday weekend. I think that as long as you get to the parks early and take advantage of fast pass, you are fine no matter what the crowds. The most crowded park we encountered (including all Disney parks and universal parks) was Blizzard Beach.... it was very crowded the Tuesday after Memorial Day Monday. On that same day, we went to MGM in the morning and were shocked that by 9:45 a.m. or so, there were very long waits for the rides.
On the Sunday of that weekend we arrived at the MK right at opening and from 9 a.m. until about 11:30 or 12 we were able to ride MANY rides with little to no waits. By 11:30 the park was filling up. By 1:00 p.m. it was very crowded. Again - get there at park openings and head for the "big" rides and I think you will have no problem.
We are going again this year for MDW..... enjoy!