Quote Originally Posted by Kitsune06
I find it has to have some small degree of 'suction' in order to completely prevent leakage. Has anyone else found this to be true?
I think I know what you're saying, but that's not exactly how it works. Once it's seated properly, you will feel a bit of suction if you try to move the cup around. But the suction is not what actually prevents leakage, since air bubbles have to be able to escape as the cup fills up with liquid.

TMI: I can always tell when I'm flowing hard when the cup is burping a lot.