How bad is it? It's a commando type movie even Steven Seagal wouldn't make.
Should you see it? It works as an action film and as camp, so yes.
The funniest thing about this movie is the sound effects; everything makes a WHOOSH sound, including guys just turning their heads. The plot has a rogue Seal planning to launch a nuclear attack from an island in the north of Japan, so retired Seals are called on to take him out, with the added note that the island is full of methane, so all fighting must be done with knives (the fact that the clanging of knives could ignite a spark is beside the point). This film has action that looks like an episode of TV's "Power Rangers" and is fun in a stupid way. Bazookas come out of nowhere. People do needless backflips while fighting. There's a paintball gun loaded with acid. There's an attack with a scarf (a scarf!) and a guy, who when impaled, shrugs it off with "It's okay. I'm fine." Though the whole island is full of flammable gas, the bad guy smokes cigars.