Before digital, gel filters were the de facto standard and first choice for 4x5 and larger format studio photography. In large format they are often used in a filter holder on the back of the lens, protected inside the camera bellows. It was common to stack up to three filters for color correction on E6 film. Gels have excellent optical qualities, even compared to the best glass filters.
Originally Posted by Poisson Du Jour
I would agree that gels are not so great in front of the lens for field work, but only because of their more fragile nature and difficulty in cleaning them, not because of optical shortcomings.