I have not yet used a filler personally but I have watched the filler-no filler debate for a couple of years on a number of boards.
Clearly, the camps are divided. The anti-filler folks say that using dacron, or corn meal fouls and eventually ruins the barrel. The pro-filler crowd says that using filler keeps the powder in place in a large capacity case, avoids dangerous pressure flucuations, and avoids poor accuracy due to bullet speed fluctuations. In fact, MD Smith's reloading board
www.reloadammo.com provides different velocities based on "powder up" or "powder down" in the case.
For myself, I am leaning toward the pro-filler folks because I know small charges of fast burning powders in a large case (like a .45 Colt or a 45/70) can create a dangerous pressure situation. The largest case I currently reload is the .45 Long Colt, and I solve the problem by using a slower burning powder (2400) which fills up the case more.
Hope this helps, Mikey