Here's an excerpt from Christopher Kimball's (Cook's Illustrated) "The Dessert Bible" on this topic:
"The Great Crisco vs. Butter Debate -- Crisco (vegetable shortening) is hydrogentaed vegetable oil -- hydrogen atoms are pumped into veg. oils -- which is a solid, stable product even at room temperature. This process creates a relatively unhealthy saturated fat, but the good news is that it works wonders with pie dough. (As for the health issues, Americans consume very little veg. shortening through home cooking, so I doubt this is much of an issue. However, it is used extensively in convenience foods, which is where the lion's share of our consumption occurs.) The reason I like to use both Crisco and butter is that the Crisco does a good job of coating the flour, which makes the dough flaky and tender, while the butter adds flavor. Unlike butter, Crisco will hold its share even in hot conditions, which makes the dough almost foolproof, immune to overworking. Butter, of course, has lots of flavor, while an all-Crisco crust will be disappointing in terms of taste. Experienced bakers use only butter, but I find that most home cooks (myself included) are beter off using 50-50 combination of Crisco and butter."
Speaking of the health issues with Crisco, over the weekend I found an organic veg. shortening by Spectrum Natural that doesn't have the trans-fatty acids of Crisco. While initially excited by my find, my enthusiasm was stemmed when I made a batch of chocolate cookies with the product. While the end result had similar consistancy and staying power as if I used Crisco, the funky oily taste is a little too much for me. If anyone knows of a good Crisco substitute, that would be greatly appreciated.