In the April issue of Cook's Illustrated they did a test on vanilla beans and vanilla extracts.
They used some major brands including McCormick whom many of us grew up with , and Neilsen-Massey that is found in many gourmet stores and then some branded ones like Durkee.
Their conclusion is that good vanilla gets "cooked out" if baked very long. They felt that if you add vanilla extract after the pudding is made you get more flavor.
They tasted different real and fake vanillas in milk, the way that professional tasters do. The real vanillas of course won the day.
One reason is that real vanilla has over 250 flavor compounds in it and they develop as the extract sits. You can taste such subtle spicy, nutty and a complex flavors and in the Fake Vanilla they tasted bitter and medicinal. No doubt, imitation vanilla is made from a by product of paper production or it is a tar derivative. They did say that for making cookies imitation vanilla can do a pretty good job.
Well one thing for sure, it's hard to ramp up your vanilla in a recipe which I OFTEN do, to get that INCREDIBLE flavor if you use imitation. It will turn things bitter fast!