Yes - we are in our 3rd year and LOVE IT. We paid just over 15k for a brand new van and it has an unlimited life. Fees are around £3.5k a year but we have reallocated our holiday fund to the caravan and tend not to holiday abroad anymore. You can pay fees in installments (around £300 a month). You can barter over the cost of the caravan but not the fees and be warned if they include 2006 site fees in the sale, you'll have to fork out 2007 site fees in November. Also, do not take their finance - we took out a personal loan (a lot cheaper). Where are you considering horseshoe?
Pros: Every weekend is a holiday for us (we get in the car, drive down from London on a Friday night) where we escape from the hassles and worries of our day-to-day life. We are able to take our holidays the same time as the school ones. The kids LOVE IT - they can go swimming/to the beach/enjoy the entertainment whenever they want. It is home from home from us - we even have a washing machine. We have piped gas so have no worries about supply (and price rises).
Also, it has really strengthened our marriage - we get to spend quality time together rather than being at home trying to finish all the jobs that need doing.
We have made some lovely friends as well, and really enjoy the community feel.
Cons: Repairs/jobs at home tend to get left (although not washing); Extra Petrol costs; no mumsnet
If you need help negotiating - pitfalls to watch out for, etc, let me know.