The original 1965 Japanese film "Gamera" http://pro.imdb.com/title/tt0059080/ was essentially an updating of the darker, less kid-oriented Gojira (Godzilla)for 1960s sensibilities. Gamera, of course, is a giant, flying, flame-throwing turtle who literally consumes energy - not quite as big as some versions of Godzilla, but generally similar in most ways.
This version of the original film was edited and recut by the notorious Sandy Frank. And just like the Americanized version of Godzilla ("Godzilla King of the Monsters"), "Gammera the Invincible" gets more than just the spelling wrong. The American scenes are not nearly as ludicrous and annoying as those added to the great Gojira, but don't really add much to the story either because there is little follow up on them.
The film starts off promising, there are a few scenes worth of character development, and there are enough personalities to create some tension outside of the main plot. Once Gamera appears, however, the film begins to descend into a fairly run-of-the mill kaiju film.
The acting is good enough- even the American add-ons are OK. The directing is pretty good for this period and genre, and the special effects are not bad at all for their time (all miniatures). Some of the sets and backdrops are actually very good.
The biggest problem here, of course, is that there is little to nothing original about this film. Gamera, however, develops a much more unique personality in his later films - most of which are worth watching if you are a kaiju fan.