Tobias Frere-Jones, interviewed at Co.Design:
It works well in big sizes, but it can be really weak in small sizes. Shapes like ‘C’ and ‘S’ curl back into themselves, leaving tight “apertures” — the channels of white between a letter’s interior and exterior. So each shape halts the eye again and again, rather than ushering it along the line. The lowercase ‘e,‘ the most common letter in English and many other languages, takes an especially unobliging form. These and other letters can be a pixel away from being some other letter, and we’re left to deal with flickers of doubt as we read.
Luckily apps and mobile sites on Apple’s platforms aren’t limited to Helvetica. For a similarly contemporary (and more legible) look for your project’s interface, consider the alternatives that Stephen Coles suggests:
Fedra Sans, JAF Bernini Sans, FF Unit, Whitney, Guardian Sans.
See also, why our choice of UI typeface is important.