I started with a 9mm filtered Molina pipe (used Vauen's charcoal filters), and so the pipes I've purchased afterwards were all filtered. After several months I accidently forgot to put the filter in a Rattray's pipe I own, and I felt like I am getting a little more taste. Bought an unfiltered Stanwell, and after giving it a go, never used filtered again. Most of all I guess it's because I'm lazy, and and it's one less thing to do.