We've gone for plenty of last minute trips and there has only been ONE time I have NOT gotten a last minute ADR. It might not always be at your ideal time, but for a party of 4 or 6, you can generally get one. I've had the best luck either calling 5-6 days prior (since that's around the room cancellation cut-off), or the day of (I have Disney Dining on speed-dial) when I call once an hour until I get what I want. I have them check all available times, and also some restaurants seat in parties of 4/8, others are 2/4/6/8. At times you can ask for a table for 4 and nothing is available, but they have one for 6 (in which case you book it and tell them 2 people backed out when you check in). I've gotten 'Ohana, Boma, Biergarten, San Angel Inn, Chefs de France, and Akershus at the last minute.