Courtesy of their 10 sec demo use of Assistant, I've learned it works with RealBand and PowerTracks in both VST and standalone. As with Keith, I think it's the easiest to use and this includes the free stuff.
A visit to Celemony's website will give you a comparison of each of their software and what you get it each.
The $69 was only until end of 2010. Now Essential is $99 unless you can find the $69 price somewhere on the net. Assistant gives you a bit more tweaking capability of the voice - but you can upgrade from Essential to Assistant.

Ian

Ian
My "Original Tunes" Site
My gene pool needs more chlorine.