They're usually very similar to the original but with plenty more detail, little/no FPS hit, and a bit more atmosphere, but I use, always have used, and for the foreseeable future, always will use Qarl et al's work in the Visual Pack, Complete. IMO, it tops all the other graphics packs out there. Now, there are some things it doesn't redo, so Vurt's trees and grasses are recommended as well as your choice of sky replacers, and whatever armor/weapons you can pick up. But for the general world textures, I think that's by far the best pack done so far. If I don't use it, I hardly recognize the game, and I occasionally mistake them for vanilla textures until I remember the originals were terribly pixelated.
http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=2915