Yes it is of course worth it. I think 15 is not to expensive. And I would only go on if it is a beautiful day with a blue sky. But you should keep in mind that you loose the I am in DLRP and I can forget the world outside feeling, because you can see how small DLRP is!
my hubby, jake 5 and libby 3 went on it in march, i stayed firmly on the ground with emm, but the kids enjoyed it and wanted to go back on it,matt on the other hand wasn't to kean as it was windy up there, but i am sure he would do it on a less windy day, oh and he stupidly forgot to take any pics, but took loads on the video, i am sure you will have fun.
Without doubt this is a must do experience. Some of our party were unsure at first.
We have declared it as a must do on every trip Here is the link to my trip report and photos of the experience
Last year we were there when it was called Globe (wasnt it?) and didnt go on it, but after seeing these positive reviews i think we mite give it a wirl in 11 days :
Oh yes it is worth it! You really get a true glimpse of the size of the park (the car-parks looks vast!) and how far away some of the hotels are. Try to go up on a clear day if you can. During our stay in March the balloon was in operation every day and began making it's first trip at approx 10am. It was still going up when it began to get dark at 5-6pm. We took an 8yr old and a 4yr old and then also enjoyed it. I am personally not very good with heights and so I held on to the bars for dear life while we were rising up. But once we had reached the optimum height (I believe 300ft) I felt steady enough to walk round the other side to grab some pictures.