I hate whenever somebody prefers something old it just must mean that they're OLD TIMERS, STUCK IN THE PAST, WEARING NOSTALGIA SHADES. Ugh. Couldn't it just be...hey, the show was actually well made and good? With some of these shows, you need to put things in perspective. If you became a Sentai fan with the 32nd entry, when you go back and watch the eleventh show, you need to keep in mind things like effects limitation, the fact that there was just a different style of storytelling in television at the time, and what had all be done with the franchise by that point. You might see nothing special in, say, Maskman, but at the time it was kicking down some boundaries.
I do think Bioman is overrated, but it's not a bad show. It's Gekiranger of the '80s. It's a good show, but when people call Geki a "gem" or say sentai died after Bioman...I raise an eyebrow.
I think Bioman took some ideas from Denjiman and Sunvulcan, slammed 'em together, and improved upon some of the ideas. I think he Bio Planet has a more interesting background than the Denji Planet, and I find Dr. Man himself to be more interesting than any of the Black Magma characters. But this could be biased, since I think Hirohisa Soda is a better writer than Shozo Uehara.
For me, a big weakness of Bioman is the heroes. Red and Yellow get the focus, and they are good, but Blue's just sort of there, and Green and Pink have the misfortune of being played by who I consider to be the only poor actors to play a hero in the Showa shows.