I made this a few days ago for my brother's wife. She absolutely loved it. I plan to make a batch soon for my father in law but using walnuts instead of pecans. I just used half honey nut and half cinnamon cereal. I did not seen the reason to be buying 3 boxes of cereal to try the recipe.
I do not have a microwave bowl that big so after mixing together I spread it onto a buttered sheet pan and popped it into a 350 degree over for 10-15 minutes to get crispy. I stirred it around every 3 minutes or so.

Chex Pumpkin Pie Crunch

Ingredients

1/4 cup brown sugar
1 tablespoon pumpkin pie spice
1/4 cup butter
2 teaspoons vanilla
2 cups Cinnamon Chex Cereal
2 cups wheat Chex cereal
2 cups honey nut Chex cereal
8 ounces pecans

Directions
1. In small bowl, mix brown sugar and pumpkin pie spice; set aside. In small microwave-safe dish, microwave butter on HIGH about 30 seconds or until melted. Stir in vanilla.
2. In large microwavable bowl, mix all cereals and pecans. Pour butter mixture over cereal mixture, stirring until evenly distributed. Add sugar and spice mixture and stir until coated.
3. Microwave uncovered on HIGH 5 minutes or until mixture begins to brown, stirring every minute. Spread on wax paper or a cookie sheet to cool. Store in airtight container.