Kids are 6 months now and I'd like to try introducing a sippy or straw cup. How did you go about it, and were you successful around 6 months or was it later that everything clicked?

So far I've tried a nuby level 1 sippy with them. They'll drink, but only if I give it to them like a bottle, and only a little bit. My daughter totally went after hubby's straw at Starbucks the other day, so I grabbed a few playtex flexible straw cups today. They'll put their mouths around the straw, but I guess the sucking instinct didn't really kick in. They are mostly formula fed, so they have no issues with bottles.