We have had them since the children were small, and I agree with what others have said. They are very expensive (starting at over £60k to buy new on the site we use) and yes, they have a 'shelf life' that means they depreciate every year and have to be renewed if you want to stay on the site.
We had to sell one fairly recently as it had reached its age limit. We'd paid £37k second hand, and could choose £10k trade-in against a new one, or £1200 to get rid of it. It was in perfect condition, and we'd had it for five or six years. The site fees are £4500 a year, plus gas and electricity. Then there are the costs of coming and going. When the children got to teenage they didn't want to go so much, and yes, there was the fact that we rarely had holidays elsewhere, as when we had time off work we wanted to get use out of the caravan (plus the financial considerations).
On the other hand, we can go as often as we like, and we do use it a lot in the summer (it is open most of the year). We don't have to pack much, as we have kitted it out, and this time we bought a 'lodge' which is a glorified caravan, but doesn't have fitted furniture, so you buy what suits your taste and requirements. Things like bedding and towels, cosmetics and some clothes live there, as well as books and DVDs, so we can basically just leave home and go, which is useful. When the children were young, I used to go with them and take their friends, as I always had more time off work than my husband, so they gave us flexibility in that way. Later on, I started to go with friends, and have had some good breaks in that way, too.
They are not like caravans of old. Ours is centrally heated and double glazed, has a bath, a shower (and loo, obviously!) and the kitchens come with mod cons like freezers and microwaves. You can get washing machines and dishwashers too, but they take up space at the expense of cupboards.
We probably could have bought a property for the cost of rent, bills and renewing the caravans over the years, but neither of us likes the idea of beauty spots becoming ghost villages, and prices rising beyond the reach of the locals. This way, there is minimal impact on the area, and the basic maintenance of the site is done by the owners in return for the rent.
So overall, like most things it is swings and roundabouts. My advice FWIW is to look for a site that has as long a lifespan for the caravans as you can get. Lodges usually have longer than caravans, but are more expensive. Also, decide whether you want family activities or peace and quiet before choosing, as moving or selling up is expensive. Think about proximity to a station, in case driving becomes an issue down the line. It might make it accessible to other family members, too. Factor in rent costs and depreciation against the number of times you will use it, and remember that if you love being there (we do) it's not a simple quid pro quo.