This may open a whole can of worms, but I like vintage knobs. They just look and feel better for my personal tastes. 6mm is close enough to 1/4" (0.25 inches) that you can use TONS of different old stock knobs. Smooth shafts are good because the knobs will either have set screws or collets. Knurled/splined shafts drive me crazy because the position where I want the pointer is so often BETWEEN the splines.
Anyhow, you can often buy big lots of old knobs on ebay or from places like Electronics Goldmine.