But still, he never showed any real jutsu. I wonder if he is strong :O
Spoiler:
Zetsu has shown a couple jutsu such as the Spore Jutsu where he plants spores within a person then drains the persons charka, then he can transfer their charka to somebody else. In the case that it was shown it was used to help Sasuke. Psychically "strong" I kind of doubt that. His real strengths rely as a reconnaissance. Or as an assistant. Most of what I have seen him do is obtain data like when he was watching Naruto vs. Sasuke at Village of End, or when Sasuke fought Itachi.