This picture from my CCSP photo tour illustrates what typical smog looks like in Denver.
You can clearly see a thick layer of brown haze that blocks out what would be a crystal clear view of the foothills. The taller peaks in the background are still visible because they are above the inversion layer. I wouldn't not live in Denver just because of this, but it's nothing to downplay either. This photo was taken last Monday at around 11:30 in the morning:
There have been some really bad days, both in the winter and in the summer, when the air in Denver is so foul that it smells like an oil refinery (and nowhere near Commerce City). Fortunately that only happens about two or three days out of the year.