We went with Adventure Treks in 2007 on their 35 day tour. While you will pay more for the caravan then if you went by yourself, I believe some destinations are covered better via a tour group. This goes for a RV group, land tour or when doing a cruise. Further it depends on your mindset. Can you follow schedules, leave a location even if you don't want to, etc. We enjoyed the group we went with, and while we mostly travel on our own, we are talking about doing another caravan to the Canadian Maritimes.
I suggest you look at all caravan companies and their iteneraries (the majority will have the same attractions) and see if it fits your travelling style. What most people comment on is the caravan either spends to little or to much time in locations.
We are also talking about going back to Alaska, and this time (as we now know what to expect) we will do it on our own.