I love to gently lick a man's scrotum and balls -- AW kindly keeps his shaved to make it an even more pleasant experience for both of us.
I don't ever "suck" on them per se...sometimes I do gently take one into my mouth and lick around on it, which he likes a lot. He's very turned on by the idea of my getting both of them into my mouth, but it's difficult to do without hurting him. You definitely can't just "shove" one in if it slips out.
But done very, very carefully, it can be hot, especially if you do gentle tongue work once they are inside their moist, hot new home. However, I'm not sure how much of the pleasure is psychological and how much of it is physical.
I have sucked gently on men's scrotums before (the portion between the balls and cock), and this seems to be pleasurable for them, if done carefully, but I haven't actually used suction on anyone's balls, assuming it would be painful and unpleasant.