This is the first time I've seen this episode in many, many years. I'd like to say it still holds up, but in many ways it doesn't.
First off is the fighting between Ralph & Bill that leads to Ralph taking of the suit in the first place. It feels kind of forced. I get that they're supposed to have been getting on each other's nerves being cooped up for 3 days on a stakeout, but that just doesn't seem to be enough to justify how fed up Ralph gets.
Next is the interior of the spaceship itself. I was sufficiently wowed when I first saw this in when I was 13, even though I did recognize the sound effects as being from Star Trek. But man, I didn't realize nearly all the sound effects were from Star Trek. The doors, the bridge noises, the warp drive... worse yet, some of the sound effects were from Lost in Space! And now I recognize the footage of the alien's homeworld as being a simulation of Mars and Olympus Mons. Basically, they just didn't have the money to develop much original beyond the alien himself and his little sidekick.
But Greatest American Hero never had a big budget, so I can forgive all that. And the scenes after Ralph has been shot and Bill is holding him in his arms are just priceless. "Maybe it's our time," Ralph says, referring to the episode where their predecessors with the suit died and were taken away in the space ship. "They'll send you down another partner," Ralph chokes out as he's dying. "I got the best partner in the world, I don't want another partner" Bill replies, and Robert Culp makes it sound 100% believable. Damn, I'm sad he just passed away. That man was a great actor.
So even though the special effects may not hold up, the characters still make it worthwhile.