Strongly recommend this plugin. The formant pitch shifting is pretty much the only way I can get a good femme vocal recording from my testosterone laden cig-y vocal chords. The rest of the Soundtoys collection is pretty fantastic, but replacable. Alterboy is the only plugin I know of that lets you pitch shift the way it does.
Alterboy does introduce phase issues. They’re easy to work around though, even desirable on certain songs. My favorite use of Alterboy on a mainstream track is Story of OJ by Jay-Z, where it’s used on the sample. The artifacting from Alterboy works in that context really well.