Wow! What a great experience! We had originally booked for December 2, but a few days before the weather said 90% chance of rain but December 1 looked really good. We were lucky that they still had available seats to change our booking because we ended up going on an absolutely beautiful day. Fiordland National Park is such a huge and remote area that is not easily navigable or viewable by land. We did the fly/cruise/fly package and were able to see it (not all of course) from the sky as well as from ground level in the cruise. It's definitely the way to go, and it saved us probably 5-6 hours of driving as we originally planned to go there driving from Te Anau.
Our flight took us from Queenstown over the Wakatipu river and through/over the mountains to land at Milford Sound. There were many beautiful sights to see along the way from snowcapped peaks to beautiful valleys. Our pilot was also our tour guide, narrating the things that we passed, and we could clearly hear him through our headphones. The windows of the plane are large and clean, so we were able to get a lot of great pictures.
The Mitre Peak Cruises boat had 2 levels, and you could choose to stay inside, protected from the wind and or water spray, or you could go out on deck to take pictures. We took Milford Sound all the way out to the Tasman Sea and back. We stopped by a couple of beautiful waterfalls, including the 2nd tallest in Fiordland Park, 146m, almost 3x taller than Niagara Falls. Our boat got very close so you could look up into the falls if you were brave enough to be out with the water spray.
Overall, we couldn't recommend this tour more. In retrospect, we'd have skipped the stop at Te Anau entirely since we ended up seeing Milford Sound through this tour.