After trying so many bottles, some of which promised to be "dripless" but failed, we heard of the Camelbak and we LOVE IT.
It's so easy to assemble, easy to clean (dishwasher safe!!!)and no matter hwo many times our kids would drop it and twist it it's still spill-free and not a drop of liquid is to be found outside teh bottle. We've even discovered, by chance, that if you forget to insert the plastic straw after washing it, the bottle is still usable by simply tilting it up and drinking (like regular bottle), as long as you place the mouth-piece on top.
We recomend it to anyone!