We were there this past Feb. the week before (actually a week and a half before) which was great! Low crowds and awesome weather (even a bit chilly before we left). This was a great time to go.
We went in Feb 2003 the Thursday before Presidents' Day weekend until that Sunday...busy, busy, busy. In fact that Friday was Valentines day and we couldn't get into any of the restaurants in EPCOT without PS's that night, they weren't taking any walk-ups. I would never go back again that weekend, but that's me. If you can't take your children out of school you have to go with what you can.
We were there the end of April/early May in 2003 also and it was a pretty good time to go, not too busy. It's just starting to really warm up then and sometimes it can be downright hot but not like summer. It's a good time to go.