From here in the U.S., it looks like 9 mm and other filters are most popular in Europe, though an earlier post on this thread calls that into question. In any case, it at least appears that filters are more popular in Europe than here. Group psychology does hold sway, so U.S. smokers tend to follow their brethren (and sisters) in not using filters as much or at all, whereas it is more of a "thing" in some places in Europe. I often come a cross groups of pipes at online retailers in the U.S. that are available at discount seemingly because they are 9 mm filter pipes, and you can get some good deals on those. I have a Kemberling Austrian bent Rhodesian 9 mm filter pipe that smokes just fine with no filter and is a beautiful pipe all around, that my sister sent me as a gift after an Austrian trip.