Its not easy to land skill shots in League of Legends. Especially recently when Riot went through some champions and nerfed their width rate. Amumu's ultimate happens to be just that, a skill shot that's the road to winning a teamfight.
 
I view Amumus bandage toss to be similar to Blitzcrank's pull except that you go to the enemy. However once Amumu gets to the enemy/s he has a wide range of options on what to do with them. If the Amumu is any good he would wait till they all bunch up and root them using his ultimate to keep them in-place.
Combined with other deadly ultimate such as Fiddlesticks or Gangplanks the damage just from the ultimates can kill half the team.
 
Too Long Did Not Read (Tl:DR)
Try to use Amumus ultimate in a way so that their whole team gets rooted in it. And if the team is out of reach use bandage toss to get to them.