Although best known for their Godzilla series, Japan's Toho Studios made many other sci fi and fantasy films. This was the first of many that would be about aliens from another world invading Earth {that idea would be later incorporated into quite a few Godzilla films}. It's dated in some respects but still pretty good fun and must have seemed quite impressive at the time of it's western release. Before that the only major film that dealt with alien invasion on a "full scale,open war" kind of level was The War Of The Worlds, and The Mysterians attempts even more, if not quite succeeding, in the way of spectacle.
The first half hour is terrific, the characters are briefly introduced, strange things happen and a large bird-like robot goes on the rampage in Godzilla fashion. After this,things go downhill a little bit, too much time is taken up with meetings and the lengthy battle scenes between the aliens and the humans are a bit static and go on a little too long. Nevertherless there is a charming toy-like nature to these scenes, as various futuristic weapons are employed against the invaders.
Special effects are occasionally poor {watch out for the awful superimposed explosions} but generally better than most Hollywood sci fi films of the time. Filmed in very garish colour {which particularly highlights the alien's costumes} and with a tremendous main march theme by Akira Ifikube, the Mysterians has it's dull bits, but is still worth watching for some childish fun. Toho would perfect the alien invasion idea two years later with the incredible Battle In Outer Space.