My big issue with the vertical nipples is the doggone dripping. They drip while the chickens are drinking, but even worse they drip every time a chick walks under the waterer and brushes a nipple with it's back. But I only use them for brand new chicks, and every single time, without exception, they learn to use it in minutes. I just think the vertical nipples take less pressure to activate, so that's why I start with them. As soon as they know exactly what that little metal trigger is for and they are a little stronger, they go straight to horizontal. There's a moment or two of confusion, but that's soon past and they don't even think twice about using it. Then the bucket with the verticals is washed, air dried, and put away for the next chicks.
I used to worry about them not getting enough water because when we let them out of the run for some free time they hit that open water dish out in the yard, or a puddle, or any other water they see and drink like they've been on a Sahara Desert trek. I have come to understand that that's just chickens.....silly things! They'll do the same thing with food - put something down and you'd think they hadn't eaten in days. So I don't worry about it anymore.