Forever ago (or two months, whatever), I mentioned my quest to find a sippy cup that won't leak in my handbag. The comments from that post were very helpful and full of wisdom and suggestions from those who have gone before. Thank you, my Internet friends, for that. I thought I would take my findings combined with those comments and compile them here for any mommies starting sippys or totally frustrated like me.
One thing is for sure: the Jack-boy stayed very hydrated through our test phase.
Playtex Lil Gripper, Klean Kanteen, Nalgene Grip 'n Gulp
The straw cups. Jack quickly picked up on drinking out of a straw (after a series of restaurant visits where I forgot to take a sippy.) As Leslie commented, straw cups leave a bit of liquid in the end of the straw so when you flip it out, beads of milk get everywhere. This is no bueno, my dears. We use it, but just at home and usually with water.
Playtex Lil Gripper. Against all odds, I picked this one up at Wal-mart. I was adamantly against this cup due to the word "Lil" in the name. Think what you will, but it's true. Despite its name, it is a great sippy as long as you have the valve thing placed just so in the lid. But as I cannot be trusted to consistently place them in just right, they cannot be trusted in my handbag.
The Klean Kanteen with Avent sippy add-on. This one looks so cool, and I wanted to love it. And I do. However. My Jack has a habit of knocking (sometimes throwing, we're working on it) his cup off of the high chair, and this dents the cup. I don't know if you can tell, but it's leaning a bit towards the Nalgene due to swan dives onto my tile floor. Also, this comes in pretty awesome colors, but they're not recommended to go through the dishwasher. I picked the stainless because mama is done hand-washing bottles. For now, at least. I liked this one because it has a lid. This is genius.
The Nalgene. Hands down the best of the best on our list for leak-proofness. I should've known this as I keep a Nalgene filled for myself everyday, and it never leaks. Never! One thing to note about this sippy is that it's a little big (as several of you mentioned) and sometimes it pains me to see my boy have to throw his head back and that tall cup up to get the last sips of his milk/whatever/whathaveyou. But he's growing into it, and it's our all-star.
This concludes my very scientific research. Nalgene for the win.