How would it be any more susceptible to tactical voting than approval voting? It's basically the same thing only with options for opinions between full approve & full disapprove.
If people give their chosen candidate 100% and every other candidate 0%, which violates the spirit of the idea (that people honestly give their appraisal of each candidate). The idea behind approval voting is not hurt as much by such tactical voting, since it's an up/down approve/don't approve thing, rather than asking people how much they approve of each candidate. Plus, people will feel significant pressure to vote tactically (for example, if I only agree with John Kerry 70%, but he's my most preferred candidate, I'll rate him 100%, not 70%).