For casual use and 4x6 to 8x10 prints, the lens is okay, but just like any zoom of any brand, the more zoom capability a lens has, the more distortion and colour abberation it will have too. For me personally, the 18-125 is my limit of the balance between distortion and picture quality, my eyes can't tolerate more distortion (both barrel distortion and pincushion distortion) than that.
Another caveat of 18-200, starting at the 125 to 200, the aperture is f/6.3, this renders the lens to be somewhat useless (at the 125-300 range) in a low-light situations. Outdoor, bright area shots such as Kilimanjaro Safari will be fine. Fantasmic will be useless. Again, this is only from the 125 to 200 end only.
Now answering your question "is the quality extremely different?". To me and my eyes, yes. However, if you're happy with the purchase, it's all that matters.
I honestly wish that I can't see ISO noise so I can be happy even with the cheapest Kodak P/S. I wish I can't see CA and distortion that easily so I can buy the 18-200 and other cheaper lens. I wish I can't see CA and vignetting so easily so I can buy Canon 17-85 IS lens.
I guess, Disney is taking care of the "Wishes" (pardon the bad pun)
Kelly