As per usual, I have a Sex and the City quote for this: "Balls are like purses. It's just a little bag, but we feel naked without it."
Personally, I think some man have very attractive testicles. My favorite male partner had a very curved penis and smooth, round testicles with tight skin over top of them, and he shaved them, and they looked really nice to me. I greatly prefer women to men, though, so for the most part, I don't have strong opinions on a guy's balls.