Although Byrne is one of my all-time favorite artists, I never liked his re-invention of Krypton and Lex Luthor...still don't. My favorite thing about the Byrne era was his choice to use ACTION as a Superman team-up book, which I thought made for some great reading.
Marv Wolfman was the man responsible for the billionaire Lex Luthor, not Byrne. Personally, that Lex makes more sense to me than some rogue mad scientist who hides himself and his mysteriously acquired resources in secret lairs. Made sense in the 40s, 50s, and 60s when that stuff was prevalent in kid's entertainment, and Superman was more whimsical. Not so much when you try to ground the series more in the real world as they began to do in the 70s.

The same thing goes for Byrne's Krypton. To me, it seems alien. The Krypton of the Gold/Silver/Bronze Ages, and again in the present, is too much of a "perfect Earth." Again, it fit into, and made sense in the sci-fi worlds of the 30s-60s, but doesn't hold up in a "real world" setting. The crystal planet didn't really make sense to me either, and I hate the crystal fortress, but the crystals as a source of energy and data storage, that works for me. But back to Byrne's Krypton. His was a Krypton that was at one point the ideal planet of the Gold/Silver/Bronze Ages, but had fallen over time. That evolution, and back story made sense to me, and gave Superman a second purpose on Earth, keep this planet from repeating Krypton's mistakes.

Bottom line is it's pretty hard to defend DC's superhero output of the late 50's early 60's, especially the Superman/Batman stuff. Seriously, everytime Supes needed a new power to defeat an enemy, it pretty much magically appeared. If he needed to produce armpit farts at a specific frequency to render the foe unconcious, no problem! Mr. Mixitplik and Batmite? Please. Granted, the first 12-18 months of the "Marvel Age" produced some forgettable stories, they continued to improve as they went along.

My biggest problem with Supes was that he was so nearly omnipotent, he should have been able to incapacitate any foe wthout breaking a sweat while simultaneously playing 50 games of chess. So they invented Kryptonite to make it even remotely possible for someone to beat him; suddenly, every two-bit hood in Metropolis had hunks of Kryptonite to torment him with. And when green K became old, up popped red, gold and even multi-colored!
I mostly agree droid, but I view the stuff from the 50s and 60s differently than other books. They weren't after the same audience as Marvel, or later, more serious books. The SA Superman is children't entertainment, and when I read it, I do so from that perspective. I don't expect Shakespeare. In that way, it holds up well, and is entertaining. Where I have a problem with some of the elements you mentioned is in the 70s when the books started taking more serious tones. Then multi-colored Kryptonite and wild, improbable solutions/powers become boring and stupid and out of place. In the Weisinger world though, they just fit, and can be fun.

I used to think Mxyzptlk was kind of stupid, but over time, especially now rereading the post-Byrne reboot handling of him, he's probably one of my favorite Superman nemeses. He's not really a villain, he's not out to steal or murder, he's entirely unpredictable, and he's off the charts more powerful than Superman. He's there because he's bored and wants some fun, and that makes him dangerous to anyone who gets in the way of his magic. He's a foe that requires Superman to think, and not just beat up on stuff. Mxyzptlk's main problem is he's a character that needs to be written just right, or he comes off stupid. If the writer doesn't get him, he doesn't work. If the writer does get him though, you can get some pretty good stories.

-Eric