We've had one for years, we keep it on a Camping and Caravanning club site and pay about £400 a year for it, it's kept in storage there so when we want to use it we ring up and for £6 they pitch it up for when we arrive and put it away afterwards.
It costs about £18 per night to use it depending on the time of year.
For us it's definitely worth it, it's only 45 minutes from home so we can pick the kids up from school on a Friday and be there sitting down by 5pm and stay until Sunday tea time, I also go without dh a lot in the school holidays.
The kids (aged 4 and 6) absolutely love it, they get new friends every weekend and spend their days outdoors playing.
We don't tend to tow it any more and for our 'main' holiday we hire a static in Cornwall, there's a few reasons, mainly i hate sitting at 60 or less all the way down, I also like having more room for 2 weeks worth of clothing and it's nice to just have a change of accommodation for one holiday.
I'd never own a static though because they lose a ridiculous amount of money, you pay over the odds in the first place, are stuck on the same pitch, huge annual fees which rise annually and when you sell you have to give the site owner a commission or sell it to them at way under market value.