i've used sterifilt filters (mainly a UK thing? from needle exchange. sold on
www.exchangesupplies.org) for filtering heroin, amphetamine (straight off the street, manky i know but it's so fucking weak otherwise), methoxetamine, ketamine, ethylphenidate. i've used them with the coloured nevershare syringes or those usual BD microfine 29g,whatever the common insulin ones with red cap are.
i find them really easy to use, not really sure what they filter out. with heroin the membrane goes dark brown/black in places, with the others no colour change. the only bad experience i have had with them is
1. putting them on. the shape of the filter seems to have changed a bit, which makes it vary needle to needle how easily it will snap on. a couple times i've bent the needle when it's popped out of my hand whilst clipping it on.
2. taking them off. because they snap on, you have to pull them apart quite hard. one time it jerked off and my hand recoiled and i impaled my thumb on the fucking needle. obviously not that big of a deal as i hadn't shared the needle, but should have really backloaded into a new syringe/needle but just used the same.
sometimes when sucking up stuff from a spoon with a flat bottom, the filter seems to form a vaccum seal with the spoon, just lean the syringe over a bit to break the seal and continue.
all in all, a product that i would rather use than not use (i.e. use cottons) as i feel it is safer than cotton (for one it's sterile, for two it doesn't give cotton fever, for three it retains less drug than a cotton), but does have a couple small issues.