Is the harmonizer on the kn7000 usable? ...

The one on the kn6000/kn65000 for all intent and purpose was NOT ...

While some kb's have fairly decent hamonizers, I believe vocalists who want really good sounding harmonies opt for dedicated harmony units:
http://www.tc-helicon.com/VoiceWorks.asp
http://www.digitech.com/products/Vocalist/VocalistLivePro.php

t.