I believe that Ducane was acquired by Weber a few years ago - so, they are different members of the same family.
I would put most Ducanes in about the same leage as Weber's newer "Spirit" series. I would consider them a few notches below the Genesis / Summit Series.
Their design is different from Weber, but still pretty high quality.
My Dad has one with a Natural Gas hookup. I like the design of it, but I'm not sure if he runs the proper regulator or not - because I think that his never seems to get hot enough. He loves it and says it's perfect, but I think that he's too stubborn to get it looked-at...
If you're looking at a Ducane because of lower cost, I would suggest looking around for a used Weber. I prefer my basic old Weber Spririt (Like a stripped-down, modern Genesis) to my dad's by a considerable margin.