Our family has gone to WDW for the past two years during our spring break which is early March. The timing has been great for us ... for several reasons... The crowds have been fine to deal with. We got fastpasses for many rides, but always got on without any wait then when we went back. Many rides we didn't even needs fastpasses. But again, it was early March. The weather is great if you like it to be cool. It makes walking the parks nice. At nights we needed light jackets ... and sometimes during the days we were okay with shorts and shortsleeves ... but sometimes kept those jackets on. However, since we have a desire to utilize the pools and waterparks more, this next time we're going when it's hotter!! It got chillly in the pools this past March and that wasn't so much fun! You also have to pack more clothes when you go in March, so be prepared! I took long pants, cropped pants, and shorts!!
Have a great time!