What combo are ultralisks good against in WoL?

I find that when coming up against a large Marine/Marauder group I often lose even when I have about 10 ultralisks.

It is only in a few situations, in late 4v4 games, that I have been able to go for ultralisks, but then they just died instantly.

I probably made some tactical mistakes, but I want to learn when to go for ultralisks and what combo they're good against in WoL?