What I like best is a mix of butter (for tenderness and flavor) and lard (for suppleness and texture). I wouldn't use lard for everything, although you probably could. For creamy custard fillings, or delicate fruits high in sugar and acid (like peaches, plums, and cherries) I'd probably stick with a lighter, more crumbly all-butter crust. But for more mellow fillings--apples, pears, pumpkin, pecan--as well for savory chicken or meat pies, lard n' butter works like a dream.
Why? Three words: texture, texture, texture. Lard gives a silky flakiness more like a croissant than your usual crust. The meatiness disappears and all that's left are beautiful golden-brown shards breaking up under your eager fork. If you think crust is just there to hold up the filling, this will change your mind.

Obviously, I have strong feelings about this issue. So why not them to the test and see how my pie stacked up against the competition at the annual apple-pie contest held at the charmingly local Sebastopol Gravenstein Apple Fair. This annual August event is a fund-raiser for the excellent Sonoma Farm Trails program, as well as an all-around celebration of the Gravenstein apple, Sonoma rural know-how (from beekeeping to sheep husbandry), and middle-aged guys in Hawaiian shirts jamming loud bar blues, all under the spreading oaks of Ragle Ranch Park. (The Fair continues Sunday, Aug 16, from 10am to 5pm.)
Now, full disclosure: back in 2001, a pie of mine won the Grand Championship prize in this very contest. I entered again last year, though, and didn't even make it into the top three. The pie world: a fickle place!
One of the perks of winning in 2001 was returning as a judge the following year. Which meant I saw, and tasted, all the things that can go wrong: proud, beautifully formed crusts burnt chocolate brown; pale, pallid crusts that shouted "I'm made with Crisco!"; underbaked apples chalky with starch alternated with fillings flavored with weird things like lime zest and nutmeg. As Fran Lebowitz wrote in Metropolitan Life,
People have been cooking and eating for thousands of years, so if you are the very first person to think of putting fresh lime juice in scalloped potatoes, try to imagine that there must be a reason for this.
So I made a plain old pie, only with lard and with Pink Pearl apples, my favorite heirloom because not only are they tart and snappy, they're Barbie pink. Except that you wouldn't know it, because their skin is pale and creamy, nothing special, until you cut inside and wham! Fuschia!
The pie I made was pretty in pink and the crust divine, but all for naught: after hanging out at the fair for 2 hours, checking out the goat-milking demonstration and the 1940s tractors, sampling the multiple apple pie/cobbler/fritter stands, admiring the many face-painted children and their mom-jean-wearing moms and/or tattooed dads all happily gnawing on enormous barbecued Willie Bird turkey legs, the announcement was made, and my pie was not among those honored.
Oh, well. It might be a loser, but it's a beautiful one, at least. For pie, the best is always Mom's, and how can a contestant know what kind of pie the judges came home to on a sunny afternoon?
Life is short, though. Bake pie for the people who need pie, and don't worry about the ribbons.