From my experience, Alex's lovely for Voiceover until you jack up the speaking rate to something fast...and by fast I mean a bit faster than average speech, cause it's quite annoying to have to read a whole webpage or something slowly. On the flip side, if I'm reading a book for leisure and feel like using the voice to read it to me at a slower rate, it almost feels like a real person is talking.

So, it's fantastic for what it is (free voice that comes with the OS, natural sounding and all)...but it's not that fantastic compared to some of the voices available out there.