Late Game Ultralisk/Zergling
Late game Ultralisk/Zergling is an army composition that allows a Zerg player to have mobility and counter attack options in the lategame vs a Protoss or Terran opponent. The composition covers major weakness that each unit has, a Zergling's low health and an Ultralisk's low dps per supply. Zerglings are responsible for dealing damage while the Ultralisks tank damage. Thus the Chitinous Plating upgrade is invaluable against late game bio army which rely on many Marines to deal the majority of the damage. Economically this build works well because the low Larva per resource cost of the Ultralisk leaves plenty of Larva to build Zerglings. Also, going Ultralisks will leave a zerg player with excess minerals to build Zerglings. This should not be encouragement to focus on Ultralisk production though. Too many Ultralisks will allow Protoss or Terran players to easily out dps a Zerg player. An ideal amount of Ultralisks to Zerglings should end a battle with Ultralisks alive long enough for the lings to overrun an army.
Teching to Ultraling vs Terran
The major issue with teching to Ultraling vs Terran is air control. If the Terran has large amounts of Science Vessels Irradiate will kill Ultralisks very quickly leaving the lings very vulnerable to the more robust late game bio army. Dropship play is also a threat to vital tech structures, but Ultraling should leave enough minerals to build extra lings and spore colonies for defense. Based on this fight for air control, Zerg players considering transitioning into Ultraling should have the resources to build and maintain a small force of Mutalisks and/or Scourge with the main army to protect against Science Vessels.
Defilers are a great addition to Ultraling depending on your playstyle, as Dark Swarm makes the Ultraling combination invulnerable against most Terran infantry. The major drawback to a Defiler, Ultra and ling composition is that Defilers slow down the tempo of play a lot. Consider building Defiler tech if you are attacking entrenched positions, but be aware that a small group of crackling and Ultralisks can take out an expansion even with bunkers.
Teching to Ultraling vs Protoss
When teching to Ultraling vs Protoss one of the most important things to consider are Carapace upgrades vs Protoss weapons upgrades. If the Protoss weapons upgrade is one level higher than the Zerg's Carapace upgrade a Zealot will be able to kill a Zergling in 2 hits rather than 3. This allows a Zealot to tank more damage allowing for other units like Dragoons and Archons to kill a Zerg player's Ultralisks.
A Zerg player should not tech to Ultraling vs a heavy Templar or Archon composition. The splash damage in this case is lethal to Zerglings, so opt to use a more hydra based composition. Reavers and Shuttles will require a Spire to counter, but is doable in the ZvP matchup as long as the Reaver hits lings rather than your Ultras.
The general mindset of Ultraling vs Protoss is to attack Protoss expansions using small groups of lings and Ultralisks exploiting the immobility of the Protoss army and the fast dps of lings. The key here is to split groups of units in small enough numbers so that if the Protoss deathball were to engage the Zerg army the Zerg army could hold preventing a base race situation. Zerg generally does not want to go for a base trade as attacking a Protoss natural is significantly harder than a Zerg's natural not to mention there are more buildings in a Protoss base.