A good rambutan has a firm and juicy flesh. It has a sweat taste, and it is somewhat similar taste of lychee fruit. The tree will actually look like a enlarged Chirstmas tree. A rambutan is best within mid season somewhere around June August as they seem to be most sweet, and large.. It is more sweet than sour. When it gets ripened the flesh separates easily from the seed. But when it is not quite ripe, the flesh sticks to the rambutan seed and the taste is a little sour overall, still not a bad taste though.