For cold drinks:
The top actually has three openings:
1) A twist-off cap, for when you want to take a nice big gulp.
2) A fold-out straw for sipping.
3) A one-way valve to let air into the bottle as you drink.
The one-way valve completely prevents the glug-glug-glug noise you get with similar bottles. An excellent innovation.
And for hot drinks:
It's a teapot with a twist: the bottom contains a filter and a valve. When your tea has brewed to your satisfaction, you plonk it on top of a mug. The weight of the pot sitting around the rim of the mug opens the valve, and the tea is strained into the mug, leaving the leaves in the pot.
I tend to go in for fruity herbal teas, and this has the added advantage of letting me see the teas as they brew: sometimes they're very pretty!
Of course, if you usually stick to teabags, a plain old mug is probably best.