It takes some understanding and practice to use the TC Harmonizer. It is not quite as powerful as Antares but works well if you practice with it to get the right mix for you. If the original vocal is not much, then the harmony will reflect that. Using the TC-Helicon VoiceTone Correct during vocal input can make up for a lot of vocal problems and give you a vocal track that is on key and note. If the Harmonizer is sounding bad, the vocal probably is bad as well.
Generally, the vocal is what kills most new songs--the voice is like any other instrument--training and practice.