We have gone during Spring Break once before and swore we would never do it again. BUT, we are leaving this Sunday for a week's vacation, lol. Now that our two oldest are in High School, we can't take them out of school anymore, and I am a huge supporter of taking kids out of school for family vacations. High school is so much different though than grade school and much harder to make up the work. We usually go in the really down times, and have not had to wait more than half an hour for a ride. Next week we plan on seeing hour long waits.
At any rate, there are zillions of people and huge lines. We will ride only the rides we really want to do, use Fast Pass exclusively. My family has been there enough times to know the best way to "do the parks" but we are bringing my 7 year old nephew who has never been there, hopefully he will not realize he is missing rides or attractions, lol.
Just don't let the crowds stress you out, take your time, do what you want to do, and most of all have a great time!