Food writers often advise you to cook leeks whole. That is fine for the slim, elegant ones that you find in summer; but the chunky ones prevalent now can be tough to cut up with a knife and fork. They are slimy too: consulting McGee, I learn that they owe this quality to long-chain carbohydrates, which will gel when chilled.
Sliced leeks, boiled or steamed, can be very watery. The answer, I think, is to slice them, soak them for a while (they may be gritty), and sweat them gently in a covered pan with a knob of butter, adding a little water if they start catching. Uncover the pan when they start to soften, to allow water to evaporate. The leeks retain a bright colour and fresh flavour.