There is always a lot of fuss about Leyland Cypress hedges. They have a reputation for being uncontrollable, for creating disputes between neighbors, for being ugly and, recently, for being disease pronone. There are elements of truth in all these accusations but, set against these, it is quite possible to create beautiful Leyland hedges which are excellent screens.
The Leyland Cypress (Cupressocyparis Leylandii is its Latin name) is a cross between two other species of conifer and is a classic example of hybrid vigour. It is a very fast growing plant, indeed the fastest evergreen with the exception of some Eucalyptus species. Even in poor conditions, such as thin chalky conditions and exposure to coastal winds, it is capable of decent growth. It can even tolerate a certain amount of shade. These benefits however can result in several problems. The roots of such vivid plants can remove much of the moisture and many of the nutrients from the surrounding soil (on your own side of a hedge as well as your neighbor's) and so it can be difficult to grow anything near to a Leyland hedge. The primary problem however results from the vigil of its growth.
A healthy Leyland Cypress can make several feet of growth a year. To contain it as a hedge needs ideally three sessions of trimming a year, in late spring, summer and early autumn. This is partly because it grows so fast but also because it will not grow again from the older, brown wood. Only the fresh green growth can be clipped effectively and this is at the end of all the branches. It is however possible to maintain a decent, although not perfect, hedge by twice yearly clipping. Anything less and gaps will start to appear at the bottom of the plants which will then look less attractive and stop working so well as a hedge.
Leylands have also frequently been used as screening. They are easily capable of forming a dense line of trees 15metres (50ft) high in reasonable conditions of soil and exposure. As with hedges, this can cause difficulties with neighbors and local authorities who may resent their overbearing effects and the loss of light and view which results. There have been many disputes between neighbors in the UK, several resulting in expensive law suits. The result has been legislation so that it is now possible for Councils to require any evergreen borderary hedge or screen to be reduced to 2m high.
Disease of Leyland Cypress has become increasingly common recently. Large brown areas in closely clipped hedges are an increasingly familiar site. In Britain the Royal Horticultural Society has carried out research into the problem ( http://www.rhs.org.uk/Learning/Research/Projects/cypressdieback.htm ) and in addition several bodies in the US have reported widespread occurrence of Canker Diseases. It is worth remembering that there are several alternatives to Leylands which, though less vivid, make very good substitutes. Western Red Cedar (Thuja plicata), Lawson Cypress (Chamaecyparis lawsoniana) and Yew (Taxus baccata) may substitute well depending on the situation. In addition there are many non-coniferous evergreen hedges such as Laurel (Prunus laurocerasus) which thrive in similar conditions to Leylands.
To answer the original question, if I were to need a permanent, evergreen hedge for my own garden I would choose one of the alternatives which are a little less vital or subject to disease. That said, Leylands can often be bought very cheaply.