Do you need to learn another kanji set ?
How do Japanese words imported from Chinese (on'yumi) match with today Chinese ?

Pretty difficult. Japanese speakers are required to know almost 2,000 Kanji, which is very inadequate for Chinese. Most important is the extreme difficulty with the tonal system, the phoneme set, and the grammatical differences.

Well for a japanese person, it's obvious that they can understand most of the writing, but not all. They indeed will have to learn more chinese characters. I'ts like someone learning spanish ,say that they already knew french, they will have to get use to that word (in this case chinese) even thought they are familiar with one that sounds the same in their language. It will just be alittle easier for them to learn chinese that's all. I hope i wasn't confusing

They have a good start. Much easier than other people.

Kanji is the Japanese word for Hanzi (the Chinese word for characters). All Japanese Kanji come from Chinese Hanzi and all carry the same meaning except for when they are duplicated in some cases. For the Japanese, I think it's just a case of learning new pronunciation of Kanji and maybe slightly different grammar.

A lot easier than say an English speaker, though I'd imagine it'd still be somewhat of a challenge because they are really completely different. A few of the Characters are the same, but still different pronunciation. And for the most part, they are going to have to learn a completely different writing system.

I don't know how he did it: My Japanese friend knows CJK: Chinese, Japanese and Korean.

To the person who said "Japanese and Tamil are exactly the same." Are they really? Then they are related? Then how come linguists never reached that conclusion? So if Japanese is related to Tamil, and Korean is supposedly related to Japanese, then is Korean somewhat related to Tamil, too?

It is pretty easy for the Japanese to learn Chinese as compared to people from other linguistic backgrounds. The reason being is that they are already familiar with a character system, which is the hardest part of learning Chinese. Like someone already said "Kanji" the word for Chinese characters in Japanese literally means Chinese characters and comes from the Chinese word "han zi". (note the similarity)

The classical Japanese language is basically the same as the classical Chinese language, this is where they got the Chinese characters to begin with. Then then added their own script to make up for the deficiencies of Chinese characters being used with a language which is not of Chinese origin.

The Chinese language and character system were created for and by each other. Japanese, on the other hand, is a language which has a much different, and more complex linguistic make-up than Chinese.

The characters in Japanese are sometimes written slightly different than their predecessors in Chinese and they usually always have a different and sometimes many different ways to be pronounced depending on the script (Hirogana) suffixes added to them. Many times they also carry different meanings from that of Chinese. Take for instance the characters in Chinese for "to force" ??" when used in Japanese mean "to learn" and the pronunciation is nothing like the Chinese.

So for the Japanese to learn Chinese they basically have to learn to pronounce the Characters differently and get used to a much different syntax and grammar system, which shouldn't be too difficult as Chinese is grammatically very simple.

Now, if you turn the tables, the Chinese will have a harder time to learn Japanese than the Japanese to learn Chinese because the Chinese language is such a simple language grammatically. The Chinese language is probably one of the most grammatically simple languages on Earth giving the Chinese speaker a challenge of complexed verb conjugation, time concepts and other aspects that are natural to most languages.

It's hard!! I know because my Japanese husband and I just moved to China and we're both studying Mandarin now. I wondered about this same question before too.
While it's true that the Japanese characters originated from ancient characters brought over from China... that was so long ago that both sets of characters have changed to the point that neither Chinese people nor Japanese people can read much of each other's writing now. My husband says a Japanese person who had never studied Mandarin could probably understand less than 10% of the characters.
I think this is similar with English speakers and Latin languages... certain words in Spanish and French are similar enough to English that we can guess what they mean (i.e. revolucion = revolution), but we most likely wouldn't comprehend an entire sentence or paragraph without having studied the language.

While there are still some basic characters that are the same (such as numbers), the Modern Chinese writing system used in China consists of simplified versions of the old characters (the PRC government intentionally changed them in order to make it easier for the masses to become literate), whereas in Taiwan (where they also speak Mandarin) more traditional characters are still used (although they are equally different from the Japanese versions of the original characters).

Native Japanese speakers do have an advantage over native English speakers in learning to read Mandarin. Since they already know kanji it is easier for them to pick up Chinese characters once they learn certain rules about how the old characters were altered to make the new ones. However, it seems that in general English speakers have an advantage in mastering Mandarin pronunciation since we have more of the sounds in our language than the Japanese do. We both struggle with the tones though, since neither English nor Japanese is tonal.

I asked my husband for a rough estimate of how much of the On-yomi readings are similar enough to help in understanding spoken Mandarin vocabulary, and he said probably around 20%

As for grammar, Japanese people think that Chinese grammar is closer to English grammar than it is to Japanese grammar. (It's actually not really, just different from both.)
The language with grammar most similar to that of Japanese is Korean.

If you're interested, you can find more info online about China simplifying their characters. A place to start:
http://en.wikipedia.org/wiki/Simplified_Chinese

i dont know abt mandarin but i know that learning Japaneese through TAMIL will be very easy as the sentence structure of japaneese is as same as TAMIL. also about 1300 words in Japaneese language are having same pronunciation and same meaning as those in Tamil language.