We also just returned from March Break. Had a 3-day multi-park pass, so we did both US and IOA; visits were all on week days. The ONLY busy area we encountered was WWOHP. We got there about 10 am on a Wednesday, went directly to WWOHP and were the absolute last people admitted before they started handing out return times. We waited about 90 minutes for the Harry Potter ride, but there is lots to see on the way in. The shops all had admission lines, but all of the merchandise was available at other shops in the park. Dragon Challenge was a walk on even at noon - but you have to have nerves of steel to ride. I have NEVER felt g-forces like that, and 7 inversions (holy sh**!!) on the easier Hungarian Horntail side. And forget about getting a butter-beer. They take longer to pull than a proper Guinness, making for very LONG lines. If you must have one, try the stand by the Potter ride, line up on the bridge side of the cart. For some reason this line was about half the length of the others when we were there.
Outside of WWOHP, most rides were walk-on to 15 minute waits early in the morning, but still reasonable by afternoon. We did wait about 45 minutes for Rip Ride Rocket at 3:00 in the afternoon, and the Jaws ride had about a 45 minute wait too at noon. MIB, and Mummy were under 10 minutes in mid-afternoon.
For March break we were impressed with how small the crowds were everywhere except WWHOP, especially in comparison to the throngs of people over at the Disney parks. Unless you are there Easter weeks, April should be pretty good.
Have a great time!