I like leeks in soups (do a Google search for Potato Leek soup and take your pick ). Whatever you decide, remember leeks are pretty dirty inside sometimes. Cut off the roots and the green part - you only really want the white. I usually cut the white part in half length-wise and then slice them into half-moons, if that makes sense. Then I soak them in a bowl of water for a while. When I scoop them out of the water to drain them I try not to stir up the water too much...I'm always surprised how much sand and stuff is at the bottom of the bowl.
I used them last night as part mirepoix for quinoa. I typically cut the stalk when it starts to turn light green, use only the lighter part. Chop the leek into very thin slices, which I submerge in a large bowl of cold water to rinse any dirt out of the layers.