Singularity grenades not useful? Nonsense. I’ve been using almost nothing but singularity grenades since the game came out, and I’ve been happy with them. Every now and then I’ll try a non-singularity grenade, desparate for some variety, but I’m always disappointed by how much less useful they are. I’m always amazed that anyone bothers to use anything other than singularity grenades, especially given that grenades don’t do much damage in higher mayhem levels.
A word of advice though: use the Quasar. As others have said, singularity grenades seem to have less reach than in BL2 in general. But, for whatever reason, the Quasar seems to perform better than the rest, and does a great job at crowd control. A close contender is also Whispering Ice (quest reward from Aurelia - I hope I can get a lvl 57 one from Earl with the upcoming patch).
Blue/purple singularity grenades don’t seem to do anywhere near as much, and neither do non-Quasar legendaries that have a singularity modifier (eg. firestorm, fish slap, etc.). But you can’t go wrong with a quasar - the sticky variety is especially good, I find.
Also, be smart about how you throw them. If you’re expecting to just lob one anywhere and find 15 enemies stacked in a tower on top of each other 2 seconds later, you’re probably expecting too much. But get in the habit of throwing out 2 or 3 in slightly different places and/or timed slightly apart, and you’ll soon become a crowd-control master.