We started using a sippy cup at 6 months and DS didn't do very well with the gerber one's where you have to suck really hard, he had no idea what to do and just chewed on it like a chew toy, then we tried the Tilty sippy cup and he got it right away and it doesn't have a valve so they don't have to suck hard and it doesn't mess up their teeth.
we really liked the Tilty he is still using it now and is 13 months old and it still seems to work really well we also give it to our 5 year old because she tends to knock her drink over by accident a lot.
good luck and you might want to give the Tilty cup a try