We were there Dec. 25 - Dec. 30, 2015. Yes, it was incredibly crowded, especially MK. We have gone to WDW during all seasons, over other holidays, etc. and we never experienced crowds like we experienced this week. IMHO, nothing compares to it. We still managed to have fun, but we spent a lot of time doing things that were off the beaten path, so to speak. We did Wild Africa Trek to avoid crowds in AK, spent time at the pool (the weather was amazing that week), DS went to the Sandcastle Club, etc. If you go, my tips would be - 1) stay onsite to take advantage of Disney transportation. Traffic is terrible and even affects bus times. We stayed at BWI and it took twice the time to get anywhere by bus than it normally does. 2) stay as close to the parks as you can afford. Staying at BWI was a vacation-saver for us because we could walk to two parks. Even the boat lines from DHS were incredibly long that week. 3) Do things off the beaten path - take a tour, tour the resorts, see HDDR and go to Ft. W for the evening, etc. 4) Don't go to DS (unless you go as soon as it opens). We went in the afternoon one day to see a movie at AMC. It was more crowded than MK 5) Don't go to MK. Ok, kind of kidding here, but it was a crowded nightmare for us. We literally could barely walk from one spot to the next. Go at RD and be prepared to leave if it gets too crowded for you. 6) Don't wing your meals in the parks. QS will be really crowded. If you are going to the parks for the day, make ADR's 7) If you want to see an evening show - ROL, CP, Fantasmic, etc. - book a dining package. Lines for standby will be really long. 8) If you are going to EP and don't want to see CP, avoid WS when the show is going on. The lines for CP are so long that it impedes walking around WS in the afternoon/evening. Conversely, it's a great time to tour Future World!
Hope that helps!