Definitely worth it. I'm in the East Midlands and have 2 boys at school (7 and 5). Since January we've already been to Warwick Castle twice, Sealife Centre, Alton Towers, Thorpe Park, London Eye and we are going to Chessington this weekend. In May half term we have booked a cheap Travellodge family room and are doing the Blackpool Tower attractions (lots of them there - tower, circus, soft play, dungeons) and Madame Tussauds and Sealife then the next day going to Manchester and doing Legoland Discovery Centre (and a bit of shopping in the Trafford Centre I expect). I love it and will definitely buy them again next year. I have to plan otherwise everyone drags themselves slowly out of bed on a Saturday morning and by the time we get going it isn't worth it! Making a weekend of it doesn't have to be expensive if you look out for deals (back in Feb we got a £19 family room at Premier Inn).
I've had a pass when the children were small, but now they are all at school I wouldn't. It was nice to go off for the day ad hoc but when you are restricted to weekends and school holidays it seemed to me I was forcing myself to use them to not waste money...defeated the point for us!