I absolutely love it when it's done really lightly. Quickly slapping, flicking, or biting my balls during a bj/hj is a gigantic turn-on. The physical sensation of it is amazing when you're in the right mindset for it -- it gets all your feeling + blood moving down in to your genitals. I'm incredibly dominant 99% of the time, but for that 1% I love letting a woman use my balls to control me, and treat me as an object.
I had one girl who would smack me in the balls, grab my hard cock and say "this is the only reason I'm with you." I still get turned on a little just thinking about that.
There's a trust thing to it too, like any BDSM. You can't (or shouldn't) do this with someone you just met at a bar. You really need to open yourself up and completely trust the person you're with for it to work, so afterwards it feels very intimate.