Product review (4 stars): Promising
I used two of these to run dryer duct over a 28'.  This replaced 2 of the 4 hard 90s farther along the length  of the run.  I was never able to personally asses how well this item worked, but I asked my renter several times and she said she had no trouble whatsoever with drying her clothes.

 I'll add a vent booster if needed.

Final note:  take care not to cut your hands assembling these, and tape them up well for a good seal.