I know there's a long-running Smallville thread somewhere and if someone wants to incorporate this into it, that's fine. Up until now though, I've avoided the thread because I had not watched all of the series. I finally finished it on DVD the other day.
I think the show is underrated because of its original target audience among other reasons. Yeah, it's a soap opera- and the most successful television rendition of a comic book character ever done at ten seasons and still ending because they chose to end it.
The show clearly underwent a huge shift in its last three or possibly four seasons. It was still a soap opera but the elements of that soap opera were more relatable to adults.
I've been around for a while and yet for me, this show's versions of Jonathan and Martha Kent, Lois Lane, Lana Lang and possibly Lex Luthor have become the ones I picture when I think of those characters. Luthor is too much in the "becoming the character we know" process to really judge him against other performances but, for the others, these actors (John Schneider, Annette O'Toole and Erica Durance) have become the favorite portrayals for me. Lana Lang was too much in the early seasons' style of being for a much younger audience for me to love too much but as she's the only long-term version I've seen, she's THE version for me and I do like how she ended up in the later material.
They dragged out his becoming Superman for too long, especially since he had done all of his most significant stuff while they were still calling him the Blur. But I think they were locked into the idea that the series would end with his becoming Superman.
I particularly liked the last three seasons. Until then, they dragged out the angst and the mistakes he made because, well, because they needed those seven seasons. But then he started maturing into a character we could think of as Superman. Besides, it's sort of accepted that we can forgive those mistakes because he's really "Superboy" until the very last minutes of the last episode.
I think they did a great job of maintaining his ethical code and/ or showing it's development. I liked how they had him come to accept both his heritages.
In one of the DVD interviews, one of the writers explained that this was, obviously, a story about how Clark Kent becomes Superman. He went on to say that, effectively, Clark's existence ends with the end of the series because it ends with Clark deciding that Clark Kent is the disguise to prevent people from realizing who he really is and Superman is the identity where he shows his true self. Presumably, in private, with Lois, Chloe, his mom, he is still called Clark and does not behave like an idiot. But for all public and practical purposes, Clark from the end of the show onward is just a facade.
So just some thoughts having just completed watching the series.