We were there this past April (20-26) during our DS school vacation week...but Easter was in March last year. The crowds weren't too bad for most of the trip, but they had Grad Weekend that week, so MK closed early both Friday and Saturday night. Sunday, MK was PACKED! I heard they even had to close the gates at one point. We didn't get on any of the rides we wanted to do again, but we did get to see Spectromagic and Wishes, so it was a good day. We also met Mickey, so my DD was happy.
For the most part, the weather was perfect...warm, in the 80s and not humid. Saturday night was a little rainy, but not for so long that they had to cancel Illuminations, and Sunday was a bit chilly. We wore light jackets most of the day and really shivered during the evening shows.
Anyway, I would recommend April. The weather is usually good, the crowds are definitely more manageable than Christmas week or June (the other times I have been there), and there are enough parks that if one gets crowded, you can always leave and go someplace else. In the two times I have been in April, I haven't encountered terrible crowds everywhere...even the day MK was crowded this past time, Epcot was completely empty. We walked on to every ride and into the restaurant of our choice without a reservation.
Have fun planning!