Just watched this as a random pull.
SO. MUCH. FUN. And really, the stock footage is only noticeable/bad when it comes to some of the stuff with Ghidrah...not even ALL of it, either, just mostly the stuff that randomly uses footage from GTTHM and Godzilla looks wildly different.
Gigan is awesome and I don't mind the stock score one little bit. The Godzilla suit is certainly worse for wear, which is a pretty big detractor.
Ultimately though the colorful characters and really good to great battle sequences make up for some of the film's shortcomings. This is certainly a top 10 film for me...maybe a top 5.