UPVC - sometimes, confusingly, know as PVCu - is far and away the most popular choice of material for conservatories in the United Kingdom. Possibly the main reason for this is cost; as a plastic it is easy to manufacture and to mould to practically any shape; it is durable and light, which makes it easy to handle. The most popular colour is white; woodgrain effects are also fairly popular but many people see these as being somewhat 'kitsch'; plastic pretending to be wood, no less!
One of the advantages of plastic is its relative longevity and ease of maintenance. Generally speaking, all that is needed is the occasional wipe over to remove dead leaves, spiders webs or bird stains. It is a myth however a plastic lasts practically for ever; every so often the surface oxidises and would benefit from a good polish with a light abrasive. Eventually the plastic will become brittle and prone to easy damage but this takes many years of normal wear and tear.
Plastic frames are usually metal reinforced; this metal is usually galvanise and corrosion problems are rare but they can occur. When this happens the structural integrity of the framework can be severely compromised and fairly regular checks are carried out on old conservatories to check that they are still structurally sound.
The biggest objection to plastic conservatories is one of taste. On a fairly modern property they can look quite attractive; an older property, particularly if this is in a conservation area, they would be totally inappropriate.