I just watched this for the first time
Warning: spoilers about film and book in this post.
It had a quaint charm reminiscent of other low-budget sci fi/fantasy/horror movies of the time. The special effects are even amusing in how unconvincing they are. For a film of this level, the acting isn't that bad.
Unfortunately I feel the producers missed an opportunity to give the film the escapist feel of the Burroughs novel. Burroughs ends his novel much better. The film makes the love story anticlimactic whereas in the book the romance is pivotal. Yes, David is separated from Dian (Dia) (thanks to Hooja's machinations), but he prepares to return and in fact does so at novel's close. Ja (Ra in the movie) survives and in fact helps David make the gilaks (Pellucidarian humans) no longer subject to domination by the Mahars and Sagoths.
This makes the film's narrative ultimately pointless. Yes, David frees the Pellucidarians from oppression, but he and Dian are not together. Humans in Pellucidar are still a fairly weak species. David and Abner return to the outer crust having only partly improved the lives of the humans of Pellucidar.
This is a severe step down from the book's narrative. (The scaled-down cliché of general destruction in the lost world--a trope repeated on a larger scale in The People That Time Forgot--doesn't help things.)
Overall, I felt the decent and at least somewhat enjoyed it. The downbeat ending, I felt, was really an unnecessary climaxto the film though.
Requiescat in pace, Krystle Papile. I'll always miss you.