I have had that done several times, although, I am always hoping one or more of the tacks will find its way through the nut itself, most guys wont go there, even though I would love it :)

As long as everything is clean sterile and is kept that way it is a lot of fun. I have not had any infections so far at all, and have had a metal in the genitals along the way!

I put needles in my scrotum and love it

A master recently encouraged me to hammer a needle right through each of my balls. It was pretty excruciating at first as you start penetrating the testicle but the feeling was awesome. Withdrawing the needle can be painful and I had quite a few spurts of my (yellow-ish) nut juice, which I hadn't expected but I thought it was really cool.

I have since repeated this on several occasions with darning needles and I'd really like to put several in each ball but it's kinda difficult to do by yourself. I'd also like to photograph/film doing this but again kinda difficult when you are doing the deed.

As has been said the important thing is to keep things as sterile as possible and I soak the needles in an anti-bacterial solution first and so far no infections. Just bruising and aching for a day or so.