Honestly this is review is not free of bias, I watched this show when I was five and I absolutely loved it as a kid. I will do my best effort to be not biased, but do take this review with a grain of salt.
The story is pretty simple. Our hero gets possessed by an alien who calls himself Ultraman and he goes around and beats up Kaiju that threaten Earth. The story does have progression despite how it might seem at first. I cannot say much because spoilers, but the ending is pretty spectacular. However, getting there can take lots
of patience. You won't miss much if you skip most of the ordinary kaiju fights.
This series was made in 1980 so don't expect the clean artwork you see today. Perspective can become pretty jarring and some things aren't particularly well fleshed out. Fight scenes are lacking compared to today's standards.
The opening and ending is amazing (bias). But they are without a doubt, pretty catchy. The sounds effects are pretty crude and old, but its pretty good if you can get past that.
Honestly, characters are pretty two dimensional. There isn't much character development, but the show really likes to bait people into the typical "He's gonna finally reveal his identity!" card. Pig the robot is pretty charming despite looking nothing like a pig nor is he visually appealing. Marume becomes less of a jerk. Other than that none of the humans particularly grow.
I wouldn't recommend this to anyone who is fed up with Power Rangers or anyone who don't like Godzilla like stuff.