When you read a block of Garamond in a classic book from the early 19th century, you feel the serifs. They guide you from word to word, but they're there and you see them and feel them structuring the text into lines and establishing the x-height easily for readability. Easier to read, but you're still conscious of the typeface.
The beauty of Neue Haas Grotesk compared to, say, Futura or Avenir is that it disappears. You cannot tell that you're reading a typeface, you're only seeing words. It's a highly readable font but with a very simplistic design. This is also evident with Lucida, but its letterforms are slightly more ornamental and wouldn't fit well on a mobile device.
There's a reason Neue was chosen for iOS 7. It's simple and clean, just like its design leader likes it to be.