Actually, I've always liked Heatran, lol.
Overall, while I don't think that Origin form is useless, I just don't think Origin form offers any real advantage over the Alternate form. Yes, Origin form has 20 points higher in Atk and SpA, but is that really enough to justify turning him from an awesome wall into a somewhat subpar sweeper (especially if you consider that there are already enough sweepers in ubers)? And like everyone has already said, base 90 Spe hurts; he ties with Kyogre or Groudon at best, and is outsped by the more deadlier sweepers that will likely 2HKO him before he can KO back or set up or anything.
Now of course, if Origin form had its Spe stat also changed with its HP stat (like everyone was hoping), then hands down he'd be completely awesome. But otherwise, is it really that differentiating by just switching the Atk and Def stats?
Maybe we should rethink how we play O-Giratina. O-Giratina is basically Gengar on steroids. It has 3 immunities and a few useful resistances, and a high-powered, diverse offensive movepool. I grant you that it is slower, but just like being Ghost/Dragon makes it weak to most moves in Ubers, it generally has STAB where the other pokemon (bar Dragons) do not. And it isn't as if 150/100/100 is suddenly horrible either. It's the same as Mew only with a fixed 100 more HP.
One other hidden benefit: O-Giratina is immune to Spikes/Toxic Spikes, so although it can't get Leftovers, it can't be wittled down by any Forretress setup either.
Lets take for example, a Will-O-Wisp/Rest/Calm Mind/Dragon Pulse set with maxed HP/Defense. After a Burn, Outrage is going to be just another attack that just bounces off Giratina short of a crit. In the meantime, you've got an impossibly bulky special wall after a Calm Mind that can hit any pokemon with a supercharged Dragon Pulse. I'm sure kinks can be worked out, but even losing a lot of its Bulk, all of its STABs, both of which hit a lot in Ubers for SE damage, have a constant boost.
Consider that Giratina's untouched SA is 276. If you only intend to use STAB, its effective stat is 331. Mewtwo's untouched SA is 344 and Dialga/Palkia's is 336.
If you were to max SA + nature, Mewtwo's is 448, Dialga/Palkia's is 439, and O-Giratina's is 446, right in between the two.
You can do the same with Attack, with O-Giratina's Max Attack natured Outrage is essentially equal to unboosted Rayquaza's
Adamant PO Outrage on Max/Max +Def Blissey: 62-73%
Adamant Rayquaza Outrage on Max/Max +Def Blissey: 61-71%.
Granted Rayquaza can DD, Focus Sash, Choice Band, etc, but O-Giratina has the most raw power, no ugly 4x weakness, and crazy, crazy defenses.