I was pleasantly surprised by this episode. I didn't expect to enjoy it quite as much as I did. First and foremost, as I've said on more than one occasion, I find that the writers have trouble when they introduce characters from the DC Comics canon into the "Smallville" continuity. All too often, they rely on the viewer's prior knowledge of characters from the source material, rather than developing the character fully within the context of the show.
I knew nothing of Zatanna before this episode, other than the obvious, and so I was pleased to note that I never felt like I was missing anything. Contrast this to "Legion", where a great deal of information was left out of the script. I was also somewhat shocked that Zatanna, a character that seems to thrive on the notion of temptation, was only scantily clad in one scene. Sure, the fishnets were sexy, but the fan service wasn't nearly as bad as it could have been. (That mini-skirt, however, was practically obscene!)
But I found that Serinda Swan was not well-cast because of her body; she was well cast because of her eyes. Those eyes are an important visual point of reference within the episode, and she had the perfect look for the character as presented. Kudos for that bit of casting, and also for everyone involved in the makeup and costuming!
I can't say that the plot was particularly inventive, though I must say that the end of the episode made up for whatever might have otherwise lacked in the story. Erica Durance did her best to mimic Alison Mack's line delivery and character quirks, and for the most part, the performance worked for me. But I thought that Chloe's character beats were more important and fairly authentic. Chloe has been flirting with the role of Watchtower for a couple of years now, and it's great to see it become official.
In a related note, it's great to hear that Clark has been working with the nascent Justice League, even if it has been off-screen. I can understand that it probably would have been deadly to the budget to try to bring everyone together again, so this at least serves the purpose of covering that evolution of Clark's recognition of purpose. And it makes sense of Clark's wish as well, because working with Oliver's team represents a major step away from the simplicity of a "normal life".
Even with the ending, this was something of a stand-alone episode, and the familiarity of the plot elements kept this from completely succeeding. But it could have been a lot worse, and the characterization and casting for Zatanna was a definite advantage.