I tend to have sort of a Love/Hate relationship with this movie. On one hand making King Ghidorah a guardian Monster instead of an Alien invader, and fighting along side Mothra, is a nice change of pace. But at the same time, I've seen these 2 monsters WAY TOO MANY TIMES in other Godzilla films. And I feel I often need a break from them. The soundtrack is great, and is something I would definitely listen to on its own whenever I get the chance.
Also, I like there's quite a few hidden Easter eggs in this film. Like some of the suit actors making brief cameos, as well as a reference to Mothra's Shobijin(aka: twin fairies), and even Yukijiro Hotaru from the Gamera Trilogy appearing as a one-off character who finds King Ghidorah encased in Ice.
The main problem I have with this, is the original idea for this film was supposed to feature Varan, Baragon and Anguirus, but then got changed to Mothra and King Ghidorah for better marketing value. And it's a shame really, because I really wanted to see the Underdog monsters get time in the spotlight.
Also, and this is just a personal preference, I don't like the Godzilla design in this movie. The eyebrows are too big, the jaw is square-shaped, the feet look really deformed, and he's hunched over too much. I get this is supposed to be an Evil Godzilla, wanting revenge for the deaths of the pacific war, but couldn't they have made him less ugly looking, and more menacing? Then again, the Godzilla suit from Son of Godzilla is much worse.
Anyway, The film as a whole is still really good, and worth watching for Godzilla fans, and is a stand-alone film, with no continuity to the other G-films. Meaning you can watch this film without needing to watch the other G-films, as a whole it's good, but not awful.
If you're a long time G-fan and wanna watch a Godzilla film with a nice change of pace from the other films, or a newcomer to Godzilla, give this one a watch. You might really enjoy it.