We bought APs for the first time this year, and decided to purchase TIW card as well. We got it in Epcot (to the left of the ball, and we had to show proof we were an AP holder or Florida resident). The PP is correct, we paid $75.
Based on our eating habits, I would buy it again in a second. We usually eat at 4 sit down dinners on a week long visit. On our last trip, we ate at Boma, Cinderella's Royal Table, Nine Dragons, Fort Wilderness Lodge, and Mama Melrose's. We got 20% off at each. With those discounts, along with other discounts here and there, we paid for the card that visit. Our next 3 trips will just be icing on the cake.
In case you're not aware, 18% gratuity is automatically added to the card. Since we tip 15 - 20% anyway, depending on service, that was a non-issue for us.
As for purchasing in advance, I would wait till the last minute, and buy it when you're down there. We purchased ours in April, and it will expire at the end of May.
Lastly, here's a link with more information:
http://allears.net/din/dde.htm