Leeks can be difficult to cook whole as the outside is often overcooked while the middle is still crunchy. Cooking them sous vide prevents this from happening as the low, constant temperature allows the leeks to be cooked exactly the same all the way through.
For a luxurious twist, add a dash of truffle oil to the bag when cooking but be sure to use a very small amount as it can be overpowering.
You could also try adding a little bacon fat or butter to the bag instead of the oil – both will give the leeks a really rich flavour.