I was able to upgrade for free since I had recently upgraded to version 3.3. The audio drift correction feature for me was worth upgrading in itself as I shoot dual sync audio with DSLR video.
Any further thoughts on Plural Eyes 3.5 ... I use Plural eyes 3.3 with mostly success but occasionally I have to fall back to Plural Eyes 2 as 3 just won't do it. I know others have experienced this. I don't do anything fancy .. just plain sync of multiple audio tracks.