We feel that asking whether it's worth it or not, depends more on what you want and also on what you have done. As others have done, we took the balloon ride when we took our Australia/New Zealand trip in 2009( with another company ), in Cairns, which was very likely the same company that Grand Circle/OAT uses. It cost half the price of the balloon ride over the Serengetti that was offered to us when we took our Best of Kenya and Tanzania trip the following summer, in 2010. Having taken the balloon ride in Australia and flying over rural areas and looking down and seeing kangaroos hopping around as little specks, we decided NOT to take the balloon ride in the Serengetti, since it covered similar terraine and the animals which we would have seen would appear sized in the same manner as on our Australia trip, but at double the price. Of course the sunrise is spectacular, and that's another plus, but we saw that in Australia also.
We are, however, taking the balloon ride over Cappodocia when we take our OAT turkey trip this spring, because the terrain is different, and the views and architecture spectacular, from what we've read and have been told. But back to this question. If you've been there and done that, perhaps the cost might be a factor, but if you haven't, then you might want to take the plunge.