We have one, and have had them for many years.
We love it, but they are a significant expense. As has been said, the site fees are high, and the caravans themselves depreciate terribly. Most of the 'better' sites insist that you upgrade after the caravan reaches a certain age, which means that anything over 10 years old is virtually worthless, even if it is in great condition.
Ours have had different advantages at different stages. When the children were little, it was great to have somewhere to go for as often as we liked, and we often took their friends away for a holiday. As they got older, it was somewhere safe for them to go on their own, and I used to go with my own friends when my husband was working (I always got more holidays than he did).
Now, we have one near to where my sister lives, and my daughter lives nearer to there than to our home town. It is great to be able to pop over and see them all, and having our own things there means that it is a real home from home.
Ours is sited near a station, which means that if we have to rely on public transport we can still get there easily, which is important, I think - a lot of sites are in the middle of nowhere, which is great when you have a car, but less so otherwise.
Overall, I am pleased we have it, but I daren't add up how much we have spent over the years - far more than if we had had really exotic holidays every year - but being able to have weekends away whenever we like is something we value (it would drive others mad to keep going back to the same place, though).
A lot of people buy them as a family purchase and share the rent. If you are in a position to do this, it might be worth thinking about? People will sort this out in different ways, but an annual meeting to decide who goes when, so that workers can book their leave in time would probably be enough.