John Wells's Phonetic Blog has a new post about the design of phonetic symbols in Calibri and Cambria. John Wells is a British phonetician and editor of the Longman Pronunciation Dictionary.
Here is what he has to say:
Calibri (like the font most of you will see in this blog) has a small cap i without the serifs it really needs for good legibility. It also has too much space before the stress mark, after the ɡ and after the length mark. In Cambria the serifs and stress mark are satisfactory, but the character spacing in the word ɡɑːdn̩ still leaves a lot to be desired.