Having always loved my (male) nipples being played with, I bought these for solo play as I was curious about how suckers would feel.
I picked these as they're small, like my nips and, although the suction was adequate, I was a little underwhelmed. I suspect they'd feel better on female tits, where there's more to suck on and more powerful, screw-type, cylinder suckers might be worth a try.
However, attaching one onto the end of my cock was out of this world! It felt incredible, with most of my tip sucked into it, it looked amazing with my foreskin pulled forward over the conical part and the bright purple bulb sticking out.
Wanking with the sucker in place was off the chart and I had one of the most intense orgasms in a very, very long time. With the added bonus that, when I climaxed, my cum was all neatly contained.
So, whilst the nipple suction wasn't what I'd hoped for, and they're not intended as a cock toy, I can't recommend them highly enough.