The 3rd of Go Nagai's mecha, Grendizer carries over much of what made Mazinger Z and Great Mazinger successful, but manages to freshen things up. Even though Kabuto Koji is present, it feels a lot more like a standalone series.
The art and the animation are much cleaner than the previous entries in the catalog, often closer to the more polished OVAs. The sound design is also vastly superior.
The silly and often annoying comic relief element is not as omnipresent as it was in Great Mazinger, where Boss is often the focus of attention for much of the episode. In fact, thankfully enough, Boss is only featured in a handful of episodes. Grendizer's own cast of silly characters such as Danbei and Banta is somewhat easier to ignore for the most part. And thankfully, there are no such absurdities as taking crows either.
Even if it's made for kids, the tone is more serious, with romantic and even tragic elements, and the characters have a lot more depth.
And this is reinforced by the extraordinary music of Shunsuke Kikuchi, one of the great Japanese composers of the late 20th century. The score is full of pathos and remarkably dynamic, often reminiscent of Ennio Morricone. The music alone gives much more depth to the series than what you find in Mazinger Z and Great Mazinger.
Of course, the series has its share of fillers but overall it is very enjoyable.
I would recommend watching it in Japanese if you do have that option, as the progression of the series makes more sense. The translated versions can sometimes feel as though they were done in batches of a few episodes, without any real notion of the overarching plot. That being said, I grew up watching it in French, and I can attest that they did a fantastic job with that version, despite some issues. I assume that the same goes for the Arabic and Italian versions, seeing how beloved this series remains in France, Quebec, Italy and some Arab countries almost 50 years later.