[kirakira]

To sum it up:

Japanese vs Korean
+ Grammar is almost identical
+ Chinese derived words almost identical
+ Korean has quite a few words derived from Japanese
+ If you know Korean Hanja, Japanese Kanji should be even easier.
+ Japanese pronunciation is almost a subset of Korean alphabet so pronunciation should be easy.
- Native Korean and Japanese words are completely different

Japanese vs Mandarin
+ Japanese Kanji is easy to pick up although more complicated than Simplified Chinese
+ Some Chinese words are the same as Sino-Japanese words
- Native Mandarin and Japanese words are completely different
- The majority of the Chinese vocabulary exists in Japanese but some are not used in everyday speech, some are used under different context.
- Grammar is poles apart
- Mandarin has about 7 times more possible sounds than Japanese but some sounds in Japanese does not exist in Mandarin.

Example:
E: I take the train to commute to the office.
K:會社에 電車로 通勤합니다. (Office to train by commute)
J: 会社へ　電車で　通勤します。(Office to train by commute)
C: 我坐火車去公司上班。 (I take train go office work)

Grammar wise, Chinese is similar to English where as Japanese/Korean are exactly the same but miles apart from the rest.

Let's check the vocabulary: (K - J - C - E)
會社（회사） - 会社 - 公司 - Company/Office
電車（전차） - 電車 - 火車 - Train
通勤(통근) - 通勤 - 上班 - Commute
As you can see, Sino-Korean and Sino-Japanese are the same, Chinese is struggling badly. 電車 in Chinese means the Tram, and 通勤 in Chinese is a very rarely used word for commute and the word 会社 doesn't even exist in Chinese.

Basically, if anyone can cram Japanese, it will be the Koreans.