I own the original 18-200, and love it. It's not the best optically, but its certainly not bad.
Quite frankly, excluding the focal range from any discussion about the merits of the 18-200 vs. another lens seems to me to be as silly as excluding image quality form a comparison of the Nikon 70-200 VR II vs. some other tele-zoom.
Focal range is the reason you buy lenses like this.
For what its worth, while I don't own the 24-120, I've read that its fine on DX. Not so great on FX. Then again nither is the 18-200 as its a DX lens. My guess is that they are optically about the same.
I probably should have mentioned this in my initial post, but the primary reason for the question is that I currently own the original version of the AF-S DX 18-200mm VR lens and I use it on my D300. My wife owns the AF-S 24-120mm VR lens and she uses it on her D60. We also have a AF-S DX 10-24mm that we both use, but I use it more than her.
We have been having many debates with regards to which is the best lens optically and this is the reason for the question.
Sorry for any confusion that I may have caused and thanks to everyone. Please provide more comments if you have them.