I've used them for:
Tanking an area then blasting the area with spells when the enemy is focusing it's primary attention on my Beefy Ultralisks.
Dropping them in a base when you've killed all the enemy's forces. Just watch then tear down the buildings.
How they compare to Siege Tanks depends how you uise them against Siege Tanks. If you can simply avoid that, with say, sapwn broodlings, then you won't need to worry. Best thing to do is create some sort of distraction and drop a couple of Ultralisks on the Siege Tanks. If they aren't grouped, the other tanks will help kill off the one you're killing, then you can get close enough to the other two where they can only shoot your other forces.
