I know you're good Todie - better than me - but I really don't see how you can consider Ultralisks a viable Tier 3 choice.
I'd love to see some replays, and I'm not saying this to 'call you out' - I do take your word for it for two reasons: 1) I know that you know what you're talking about 2) Blizzard hasn't done anything to the Ultralisk since the dawn of the beta, so maybe I'm missing something.
It just seems to me that Terrans don't even need to kite Ultras to kill them. Just focus-firing with Marauders is enough to get the job done really quickly.
As a Zerg player, discounting morphing them into Brood Lords, you wouldn't normally build Corruptors unless you maybe saw that your Terran opponent was getting Battlecruisers. However, Marauders aren't like that - they're something that the Terran player would have in abundance, regardless of whether or not he knows you're getting Ultralisks.
There's also the fact that Ultralisks are quite cruddy without both their upgrades, which take quite a lot of time and resources to acquire. The +2 armour upgrade is mainly to protect Ultras from Marines - the unit they're supposed to already be good at defeating. It doesn't make too much difference against the high-damage attacks of the Marauder.
To me, all these factors really makes Ultralisks ineffective :[. It's a unit that I really *want* to like though, but it's not easy to.
EDIT:
I've just been running some tests in the editor. Mind you, they aren't very accurate because it is just based on a-moving without micro, after all, but I do think that they give a rough indicator or how ineffective Ultralisks are.
I had two Zerg armies I used to compare:
3 Ultralisks + 12 Hydralisks (Total: 2100 Minerals 1200 gas)
OR
16 Roaches + 12 Hydralisks( Total: 2400 minerals 1000 gas)
I actually consider the Roach army cheaper because I value gas very highly.
So first, I sent both armies up against a pure Marine/Marauder force. The Ultralisk army fared worse.
Next, I removed some Marauders and Marines and added some Siege Tanks. My reasoning was that Marauders would do good damage to large, single targets like Ultralisks, but maybe with the Siege Tank's splash, using Roaches to tank will be less effective than Ultralisks. However, I was wrong - the Roach/Hydra combination did quite a bit more damage than the Ultralisk/Hydra combination.
Finally, I made a mixed Protoss army to go up against. It had Zealots, Stalkers a couple Sentries, but most importantly: 2 Immortals and 2 Colossi. I find that 2 Immortals is all it takes to be quite a roadblock against both Roaches and Ultralisks. Again, I wanted to see if the splash damage from Colossi made Ultralisks the better choice in this circumstance. However, the Roach/Hydra army performed much more effectively AGAIN.
Pound for pound, Roaches seem to be the better tank than Ultralisks. The only circumstance I can really see Ultralisks being useful in is in some dream situation where the Terran built only Marines without any Marauder support :[. And even then, Banelings would probably be more accessible and maybe even more effective than spending all that money and time on Ultra tech,



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. To me, Ultras and Roaches both fill a similar tank role and require at least one other unit to synergise with them. Besides early-game ZvZ or PvZ, I'd never send Roaches in by themselves.