I never use flex duct as my trunk line.
Will run flex to branch outlets, no problem
If my flex run would be longer than 15 ft, would probably run some 'oversized' hard pipe and then reduce down to flex close to boot.
Flex only used in attic also.
All hard pipe, under house.
And I personally don't like ductboard so I don't use it.
As already stated flex was originally designed for connection from a metal trunk to a diffuser a few feet away. It does create more of a pressure loss than metal duct so that must be taken into account when sizing. I too am of the less flex the better persuasion.
Engineers like to solve problems. If there are no problems handily available, they will create their own." Scott Adams
"We can't solve problems by using the same kind of thinking we used when we created them."
I like flex too... Cut a piece a few feet long, loop it so one end is pushed inside the other (so it makes a tight circle with no hole in the middle), paint it orange and ya got a pumkin!
I think flex has it's place, but the only place I actually like to use it is on the comercial jobs to hook up the diffusers to the duct, or in some cases in residental where hard piping from the duct to the register would be impossible with out a lot of elbows.
Personally if I have the choice, I never put flex where it can't be reached once the house is done (between floors or other similar places). In attics, I might use a few feet, crawlspaces only if there's absolutely no other way. Our code says 7' max, I try to stick to 3' myself.