Our first trip was last September and the crowds were l-o-w! I have video of us walking off of IASW and we are the only people in sight. Granted, I have only been there once and I have nothing to compare my trip to, but after reading some TR and seeing posts, we will always go in September, at least as long as DS is not in school. Even after he starts school, we may take him out for a Disney trip.
I did go once in March years ago and hated it (boo hiss, I know). But that was before I knew what it was all about and long before FP.
We are going in two weeks and I cannot wait!