I own them both -- bought Harmony EFX, then upgraded to Harmony Engine EVO when Antares briefly offered a special price on the upgrade. I'm just getting started with Harmony Engine EVO (using the Audio Units version with Logic Pro) but so far, I'm impressed with the sound and capabilities. Previously I'd tried an older Digitech hardware unit, the TC Helicon Harmonizer that shipped with Windows BIAB, and Melodyne's Celemony (which I like for pitch correction, but creating three-part harmony in Melodyne is not quick or easy, whatever the YouTube videos say). Yes, I don't like the Ilok, and setting it up and installing the requisite software, especially on several different CPUs, can be a bit frustrating (had several freezes and/or crashes along the way, and the copy protection scheme caused me some initial problems with other programs as well.) But now that everything is running, Harmony Engine EVO gives me the best results of any harmonizing option I've tried so far, in the least amount of time. A review of the Antares Avox2 bundle in the Dec. 2008 issue of Sound on Sound (http://www.soundonsound.com/sos/oct08/articles/avox_2.htm) said the Antares Harmony engine (included in the bundle) is "about as good as it currently gets for automated backing vocal generation." That pretty much sums it up -- bearing in mind that "as good as it gets" still isn't great, but good enough in a lot of cases. I'd have been quite happy with Harmony EFX (same sound engine) if the upgrade hadn't been cheap, and I see the EFX version is now available online (from BHPhoto, GuitarCenter, and others) for $49 -- of course, you'll also need the unfortunate Ilok (about another $40) if you don't already have one.