We have done 2 in America, 2 in South Africa and also Australia, part of Germany and a Prague, Budapest and Vienna. Some were better than others.

Depending on where you want to go try and find a tour that doesn't go to different hotels everynight. We've found some that give 2 or 3 nights in one place which allows time to recharge the batteries. A good tour guide will often have lots of additional extras you can do. You see lots with knowledgable guides and can often have experiences you had never thought of trying. We had the opportunity to ride an elephant through the bush for an hour and not seated in a basket but straddled across it. To make matters worse/better a thunder storm started up. We even crossed the Zambezi river on them with the baby elephant using its trunk to hold onto its mum's tail.

There are obviously some downsides like being herded or going to places that you may not wish to visit. Places you'd like to stay longer at. Ages on these trips can be mixed but the norm is usually 50+ but have been on some where younger people have been on them.