leeks are a lot milder than onions and absolutely awesome as a cooking ingredient. They're more like an actual vegetable than a "flavour" like onions, really. Very popular in French cuisine. When it's time, try out these recipes:
or just add to a potato soup or squash soup (or add some squash to the above recipes - they do really well together)
As regards cultivation, leeks grow a lot deeper than onions - depending on the variety (some are giant, some relatively smaller), but generally a very tall/deep-growing vegetable. Soil requirements are similar to onions: well-drained soil, though for leeks you will need a good amount of fertilizer. I would recommend sowing in small, deep pots and transplanting when the little plants are well developed. Weeding (or mulching) after transplant is crucial. Leeks grow well in association with carrots (if you're into companion planting), and can be consumed when their diameter reaches 3 cm (but depending on variety they can grow much bigger - check that out on your seed pack and on the Internet). Hilling is recommended if you want to increase the amount of white, tender part (but the green part is edible too! everything except the roots and the more fibrous outer leaves).
Hope this helps! Available for further advice.