Anyone used avent or gerber sippy cups?

i went through a lot of bottles before my son decided on the playtex drop ins. Therefor I have a bag full of Gerber and Avents. I saw that both companys make a sort of sippy cup. It looks as though there are attachements that just go on the bottle. Is that right? If so, has anyone tried them? If they dont work with the bottle or are crap I will be donating them. They are in my way.