I've used Vocal Sync quite a bit on a couple of projects. It works great, and to my ear produces very musical results.
I do find it produces more artifacts the further the vocal being aligned has to be moved, so I don't use it on the main (read loudest) vocal track where these would be especially up front, and I never try and over do it where a cut and move would be a better option.
When an altered vocal is soloed, the artifacts can be heard, but once it is combined underneath the guide track, it's very difficult to discern amongst the other FX. To my ears, the artifacts are not as noticeable as say auto tune, or pitch altering, on the same track.
The best result I've had was on an A Capella song where all the voices were singing the same words, in harmony, or double tracked with each other. I was really able to tighten up the beginnings and ends of lines and phrases, and the artifacts were almost unnoticeable underneath the lead vocal which was used as the guide track.
A great addition to Sonar and one of the best FX to be added to Platinum this year imho.