For the HVAC guys doing new construction, I know they do good stuff these days with the flex. I still don't like it because of the restricted and turbulent airflow through the flex which limits the output from the fan to a percentage less that the rated CFM's.
Regardless of the flex, when we do new or reno installs I've stuck with the old method I've used for years with white PVC and really good roof vents that don't make noise in a good wind outside. There is no corrosion and no condensation problems, and yeah, it costs a bit more, but a good explanation with the HO and I've never had an argument or complaint down the road.
Call me a radical for going overboard with installs and unconventional methods, but it works.
