FWIW, I've lived in a converted office building in Seattle. It was actually pretty nice, much nicer than most purpose-built apartment buildings. When they did the conversion, they re-engineered the interior pretty heavily and re-purposed some existing elevator shafts for other purposes that weren't required prior to the conversion.

Some office buildings will be more amenable to conversion than others, so part of this would likely be selecting buildings for conversion where the conversion is both straightforward and will produce nice apartments. You want buildings that have many floors but relatively small area per floor.