I can't speak for the Rift players but in ARAM he is extremely powerful. He has two major qualities that make me wish he would get a rework. One is if he builds Rod of Ages and one or two tank items he becomes nigh unkillable and can out tank a Galio or Malphite with Banshees, Randuins and all the other heavy hp/armor/MR items. He also ends up doing more damage because of the ap from RoA, Liandry and Rylai. Singed is just innately tanky more than almost any other champ in the game making him survive 3v1's without breaking a sweat. The second thing I hate about Singed is that his kit is invulnerable against 1/2 the champion pool in 1v1 (particularly on ARAM) His slow, high movement speed, constant poison dmg which scales very well against any adc/mage/assasin/brawler, the throw to keep you in the poison/do heavy damage, and his extreme tankiness lets him chase characters like Brand indefinately while taking almost no damage. He's a nightmare to play against because there is no hope for escape unless your own champ has a combination of two of the folowing. Higher move speed, good disengage abilities, and a lot of cc.
TLDR - I play ARAM so disregard my post Rift players.