How useful are the kanji in reading Chinese?
Personally I found learning Chinese after Japanese a very positive experience. Indeed having a good knowledge of Kanji made a big difference and eased the learning of Hanzi. At first it took me a while to get used to simplified characters. Knowing radicals has helped me a great deal. Also I tend to use a lot of technology that was not available when I was learning Japanese. Particularly for reading a writing Chinese.