While I've only seen the dubbed version and would certainly enjoy it more in its original form, there's not much saving this movie from being pretty terrible in just about every arena.
The weak points have already been addressed--lazy writing, crummy effects, and an overall lack of material to hold interest really kill this one for me. And what the heck is with those weird noises for the beams? They have a very "electronic" quality to them, which hurts the already poorly-done attempt at an illusion that the monsters are actual living beings.
Though I, too, like the idea of an evil counterpart for Mothra. I think it's a pretty cool concept, and Battra could have turned out great...too bad he didn't. I do think his larval form was executed reasonably well, and the underwater fight was pretty cool too. And as with every other Heisei movie he scored, I think Akira Ifukube's music makes the film sound a lot better than it actually is. For my money, it's a solid score and a nice listen on its own, separated from the awful story and visuals.
When I first saw this at about age ten, I thought it was one of the coolest things ever, but it hasn't held up too well at all. It's a shame, because it definitely had some potential, but the filmmakers obviously weren't willing to go anywhere with it.