I've seen this series back in 1998, and enjoyed it quite a bit!
I liked that Ken Ookabe (who played Dr. Akunomiya in Denjin Zaborger) was the villain in this one! The rest of the cast is good as well.
The SPFX, on several levels, were very well done (considering this is Toho producing the series). Guyferd transformation was quite imaginative, for a CG effect. I thought the henchmen were pretty nifty (especially the cool wraparound helmets)! The transformations of humans into Mutions was very convincing (especially the mutating effect, which was really chilling).
The martial arts here was Japanese karate. I was surprised at how much more grounded the fight scenes were, compared to Toei's stuff from the time! A bit more realistic, even though speeding them up a bit could make it better. The costumed fights fared far better than the human fights, which look a little better than Walker: Texas Ranger chop-edit fight scenes.
IIRC, master suitmaker Fuyuki Shinada did all of the costumes on the show! The designs of the characters were very impressive. It's all like something you'd find from an anime series at the time! Guyferd's design was, and still is, a breath of fresh air.
I like the fact that this series was never meant to sell toys, as it was a straightforward action series! Very well played out. The Chouseishin Series is even better, but I am still very impressed with Guyferd, for taking the Japanese superhero formula to more realistic levels. This was Toho's much-needed return to the genre.