The C-5 is extremely complex and expensive to maintain and operate, even after it received a major upgrade, including new engines, in the form of the C-5M. But its capabilities are irreplaceable for the USAF. You can read all about what it is like to fly the C-5 from a pilot's perspective in this previous special feature of mine.

Just one anecdotal personal experience I had with the C-5 that captures its unique heavy lifting abilities—a USAF Special Tactics squadron was loading up on a C-5 to head to Hawaii for some extensive training. I asked the officer that was overseeing the operation if they were happy to get a C-5 for the trip. He said that they were elated, with the difference being that instead of two or even just one C-17s, where they would have to repack up all their gear in pallets and then unpack everything once there, while also having to acquire vehicles to tow their boats and to get around at their destination, the C-5 allows them to just load up their trucks with their trailers full of gear attached. Once at their destination, they simply roll off in their own vehicles and get right to work!