This episode has a lot in common with the previous one, Divide and Conquer - at the advice of a scientist, Superman mutilates the laws of physics to develop a new superpower. I enjoyed that episode due to the fact that the new superpower was actually really interesting and it seemed Superman was actually presented with a cool challenge, and the backdrop of the episode and overall plot line were decent and fairly creative. Here, however, there's nothing to make up for the lameness and suspension-of-disbelief testing. Other than the soft science fiction twist, this episode is just about as unremarkable and predictable as a Superman episode can be. There's nothing to justify the annoyance of the obviously already incredibly powerful Superman gaining a new power. When Superman encounters a problem which he can't solve with his powers, I want him to get creative, maybe do some investigative work, not pull something nonsensical out of thin air.
Another problem is that it was kinda hard for me to believe the alloy the cube was made out of could actually be that indestructible. Even just one stupid little throw-away line about the criminal finding a chunk of metal hit by mysterious space rays or about how some great scientist discovered it and his work was stolen or something would've made a big difference in terms of believability. Even a lame explanation would've been a hundred times better than just basically glancing over how this amazing cube came to be so amazing. What matters to me is that you follow the precedent of your own universe. Superman not being able to just punch through it was interesting, sure, but it was just hard to believe as it was presented.
Another issue my sister pointed out was that, while they repeatedly reiterated how the bad guy committed a million crimes, the way they didn't get into specifics made it hard to really hate him...which is a problem, considering that if Superman succeeds, he has a DEATH sentence waiting for him.
Also, I know this is a recurring problem in this series, but the criminals just made too many stupid decisions. Also, Lois and Jimmy kidnapped? Wow. Never seen that plot point happen before.
I guess there's nothing hideously objectionable about this episode, so I can't say it was completely horrible or anything. And I guess the villain's plan and Superman trying something new with his molecule structure was mildly interesting, even though both things were also extremely lame in some ways. So it's certainly tolerable watching, but it's far from great, to say the least. Sure, this series is always flawed, but it usually has a bit (or a lot) more redeeming value than this episode presents us with. I wouldn't say you shouldn't watch it or anything, though (if you're into this series). It was far from irredeemably bad, it just wasn't very good.