Dont you find that the metal ones take ages to draw through and soon start 'bubbling' meaning you have to go and strip the thing down?
Maybe the one I had was faulty, but I have developed a real hatred of the thing, having to adjust the dose halfway through dosing after I have noticed it isn't doing it properly, leading to paranoia about how many have been underdosed, plastic one seems to draw through quick (useful when doing hundreds) and I now dont have to worry too much about volume of dose .