We went during Ontario March Break in 2006 and 2007.
The weather is nice. Everyone doesn't have Spring Break at the same time, so the crowds aren't too bad.
Here are some tips if you are staying onsite:
1. Use EE for mornings only, but head to another park at noon.
2. Don't go near parks with EE in the evening.
3. Go early for rides, take a break for swimming and resort hopping, then return to the parks in the evening for parades, shows & fireworks.
We book on our own and buy CAA
travel insurance [only $55 for my family of four and my dd has a heart condition].
Here are some tips for self-booking;
1. Consider JetBlue or Southwest from Buffalo....very cheap and good flights.
2. CAA gave us 20% off our Disney Resorts...EVEN DURING MARCH BREAK!
3. We rent a car in Florida so we have freedom and can do some off-site meals....booking a car online doesn't require a credit card and you can cancel anytime. So I book cheap cars and cancel if I get a better deal elsewhere.
4. Purchase multiple day hoppers that don't expire if you plan to return to WDW. I bought 10 day hoppers w/ 6 plusses that don't expire. That will last us 3 to 4 trips. I bought them with the good dollar.