Sometimes (not normally) the rate is lower than AAA or my corporate rate. Sometimes the rate will say it includes a 2 p.m. checkout based on availability (one time I did see in Seattle where is said 2 p.m. checkout and wasn't based on availability). If it says based on availability for the 2 p.m. checkout I wouldn't book a room with the rate and depend on it since it's a pain for me to get a 2 p.m. checkout as a diamond.
I think the AARP membership is worth it though. I get my money's worth out of it just with the Dunkin and outback and Denny's discounts. I've found the AARP discount to be a lot more valuable at IHG properties and a Ramada I stay at in LA more than at Hiltons.