The thing people are QQing about, is that they chase him when he's at low health (leaving several of their own teammates to die to singed's teammates), thru his poison cloud and towards his own team/under a tower. He then turns around, flings them into the tower or his waiting teammates, and throws down his glue so they can't get away. Free Kill.
I've done this more times than I can count. It never gets old. I have literally done laps around a tower while some moron tries to hit me. I've gotten double, even triple kills as singed doing this.
It's not because he's overpowered. It's because the average LoL player doesn't understand how to deal with a tank that can kite and deal damage at the same time. Put ghost, boots of speed, and FoN on Amumu and you have the same equation.
+1 for banshee's veil as a counter to him as a carry. If you're tanking and he flips you, the correct way to deal with it is to use it to counter-initiate on the enemy team. Singed's only ability to peel an enemy off his won carry is that fling. If it's on CD, his team effectively doesn't have a tank anymore.