Here in Taiwan, I meet tons of Mandarin speakers learning Japanese. In general, I'd say it's pretty easy for them. There are a limited number of sounds in Japanese only a few of which pose difficulty for Mandarin speakers and the kanji are a slightly simplified version of Chinese characters.

There are a lot of cognates between Chinese and Japanese, there's a good deal of overlap in terms of history, food and religion and best of all there's nearly as much Japanese media available for those who want it as there is English media.

However, the Japanese people I've met here studying Mandarin usually have a tough time of it. They can usually read pretty well, but their pronunciation and general communication abilities tend to be far worse than those of the students of every other nationality I've met at the language centers. The difference was absolutely shocking when I first started studying here.