The under the lens kind? Of course they're a dust magnet but they work. I have an old set of DuPonts that get really low, the yellows being deeper than the Ilford or Kodak equivalents.
I use filters in printing. I have Ilford 11x11 filters cut down to 8x10. With my 8x10 Elwood enlarger they are put on top of the glass in the negative carrier.
I cut the #3 smaller than I should have so it goes under the lens. No fancy filter holder for it. I just use tape to keep it in place.
Jack
I use the big Ilford filters which go above the condenser in my Beseler 23CII. A great investment and simple to use. My understanding is that they are less of a problem for dust when above the condenser. One of these days, I'd really like to get a dichroic head, but for the moment, the filters are groovy.
My ancient Beseler dichro head on the 45MXT does not have the magenta capacity needed for a lot of things, so I supplement it with the old style filters. For 35mm I ususlly use a Beseler 35 which has a convenient holder for below the lens filters. Because they are so handy, I use an Ilford filter set there.
I use a six inch set of Ilford filters under the lens in a 12x12 cold light Durst 138S conversion for 8x10. 4x5 negatives and smaller go through a dial in variable contrast Saunders 4550XLG/VCCE Enlarger.
I use the big Ilford filters above the condensor in my D2V. Since the filters are just below the light and far above the negative, they'd have to be downright filthy before the dirt would cause a problem. The Ilford filters are one of the very few things I bought new in my darkroom, and I've never regretted the investment.
I'm sure that under-the-lens filters could be fairly dusty before they would cause a problem, but I know I'd obsess about keeping them clean.
I use both a color head and Ilford filters on my 2 1/4 enlarger. Recently found a brand new set of 150mm Kodak filters (PolyMax II) at a local store for $30 for my D3 4 X 5 enlarger (condenser head with a filter "drawer"). Rebuilding a color head for this unit and will then see which way I go.
Like the filters more than the color head but the later is easier to use.
I use the Ilford filters above the condenser. In high school, we used the ones below the lens and the smallest scratches showed up very easily. I could tell which filter I used on which print just by knowing the scratches.
My Durst Modular 70 has a VC head and is a delight to use. My Laborator 1200 has a multigraph head and is a complete pig to use. Anyone want to do a swap for a devere with VC head (I'm not joking)
I have a set of Ilford filters that fit the filter drawer of the enlarger but I've just about quit using them. For the most part, my negatives print well without filtration. I've never used those that fit below the enlarging lens. My previous enlarger had no filter drawer but it was easy to pop off the head and place them on the condenser.