I change them when I need to, I went out looking at new cars about 18 months ago when I got a new job, what stopped me changing was the fact that other than being a bit newer, I wasnt really getting anything for my money.
Work have just given me a new Astra to drive around in, What does it offer over my 14 year old festa?
The astra is slower despite having a bigger engine, its a bit better on fuel, the steering wheel seems to be made out of duplo bricks and not out of the same soft touch stuff as the festa, and its a lot noiser as well.
Obviously its got no rust and things like that, but on a practical level (and this applies to all cars in my respect not just vauxhalls) I dont see what I would gain buying a new car just for the sake of it for my private use.
The wife on the other hand, she wants a new car for her, so thats why I am putting myself through the torture of car hunting at the moment, I ******* hate it.