Even people who don't like Oblivion tend to say that Shivering Isles is actually good. These people are full of shit. It's visually interesting (design-wise), and the plot is at least novel, but it's still Oblivion. Everything still looks like shit, the NPCs still look like potatoes, the writing is still awful with their idea of madness being just making Sheogorath act ever so zany (with godawful voice acting), the combat still amounts to hitting everything a billion times before it dies, and everything is still level scaled. If you can get past that, then it's better than the base game, but that's just because it looks different.
Knights of the Nine is just a mildly interesting quest line added to the base game, so all of the above minus visual interest plus some okay writing (and, yeah, the armor looks neat).
But it's still Oblivion, and Oblivion is still fucking terrible. You can try modding it, of course, but I guarantee you'll have more fun looking up and downloading mods than actually playing the game.