I think for traveling in the other's country and reading signs like "restaurant" "tea" etc, the correspondance is pretty high. Nouns and basic verbs in general are pretty close. Once the sentence gets more complicated however....
Chinese grammar is almost all done by word order, whereas Japanese grammar, which uses alphabets Chinese would be totally unable to read, would be utterly indesipherable for chinese.
Also, the new modified chinese is different from traditional chinese--which is what is still used in Japan and Taiwan. The word for the language Chinese is 中国語 in Japanese, but 汉语 in Chinese. The last character is the same in each, but as you can see they are written differently. In this example, a Chinese person would surely understand the Japanese, but I am not sure a Japanese would understand the Chinese.
I am just starting to take Chinese and my Japanese is helping a lot with the characters, but aside from that and the occasional characters that are pronounced similarly, not too much in common.