It has the benefit of being one of the most Kauai-intensive movies ever filmed. Not only was 90% of the movie actually made on that beautiful Hawaiian island, but it's actually SET there, too -- and there are very few movies both shot AND set in Hawaii.
The photography is excellent; the score is very good (but would have been better with a little more island influence), and the use of the Kauai locations, though limited to just a few, is first-rate. Garner is fine, and Culp makes a good, if foggily-motivated, villain.
However, the attitude toward the Hawaiians is painfully paternalistic, and there's a stupid, TRULY stupid, subplot involving "sorcery" and some extremely improbable caves, full of Tahitian statuary. Very bad idea.
Historically, it's a bit muddled; much is made of the idea that no one has found a way to load cattle aboard a schooner at this time. (Somewhere around 1870, I guess.) But over on the Big Island, this problem had been licked, and the founder of the Parker Ranch was already getting rich selling King Kamehameha's "big dogs.
If the script had been better, with a less predictable story, THE CASTAWAY COWBOY could have been a little gem. As it is, it's mostly a pretty time-passer.