Non Robotic Harmonies with Decabuddy or UltraPitch??

[Northwinds]

Hi all,
Trying to put in some basic vocal harmonies (low in the mix)on a few songs and have elected to try Akai Decabuddy and Waves Ultra Pitch.
So far, after a days tinkering, all I am coming up with is very "Robotic" synthesized voices.
There are mp3 sample downloads available from the Akai site, which sound very natural and exactly what I'm after, but essentially nothing like what I'm getting.
The UltraPitch has a better rating across the forums I've checked, but even after poring over the manual, I have found nothing to point to satisfactory techniques and results.
Has anyone used either of these or any thing else (plug-ins wise) and managed to achieve worthwhile results?
If so can you please give me some tips for increasing the realism?
Many thanks,
Northwinds.