How reliable is a 15-day forecast?

Ok, so I live in Texas and have had about enough warm weather and am just curious how seriously I should take a 15-day forecast, like on accuweather.com? I am just a little wary because I remember the week before Thanksgiving I was looking at various weather sites and they all said the high would be like 75 in Austin/Dallas and turns out come Thanksgiving, the high was 45 and the low was 30! I would say they were more than a little off. So, am I right to be skeptical?