They are really good! Kiwiberries are like miniature kiwis with a smooth skin (so you can just pop the whole thing in your mouth). About the size of a grape, they are more flavorful than kiwis - they are sweeter and also more tart at the same time.
I thought they were really addictive. According to the website of the one farm that sells this particular variety of kiwiberries (called "Passion Poppers"), "[t]hey are unequaled in taste, blended flavors of super sweet kiwi and exotic melon."
The flavor is definitely unique - sweet, tart, and refreshing at the same time.
Here's another picture to give you a perspective on their size:
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