We were there from the 5th thru the 13th, right before Marathon weekend and left right before the MLK holiday.
Crowds were slightly heavier than we had experienced in Jan 2004, however, they are still really light and there were few waits. The exceptions (for us at least) were at Soarin during EMH and BTMRR on a weekday which we thought was a little odd.
DH & DS swam twice, it was too cold for me, but the days were nice and sunny. We wore lightweight fleece jackets most nights, one evening we needed gloves and caps because it was really breezy.
I'd go again at this time of year, this was our second time going and it was fine. If it wasn't for our school schedule & the testing that DS missed we'd do it again, HTH!