dont let anyone fool you into thinking theres a difference in smoke taste. as for airtightness? maybe once when i tipped the glass over did a lil bit of water 'drip' out.
i have never heard the suction sound come out from the rubber seal alerting me of it not being air tight.
i have 3 grommet style tubes.
as for the milk-ability, is only because of the smaller stem diameter. theya re offered "usually" in 9 and 12mm. ut i dont see why a blower has to put a ground joint on a glass rod. know what i mean?
the size of teh grommet is whats limiting anyone from making a grommet style 18mm,, tho i suppose if you found 'ABC' gum, that woudl work just as well.
they tend to chug too,, but thats also because its rare to find a diffused stem.
nothing a local artist couldnt help you with anyway.. na'mean?
hopefully this post clears up GROMMET vs GONG confusion