-Scourge Strike builds do reasonably well on single/double boss fights, but are noticeably behind Frost DW and Blood 2H. They are superior on big aoe fights.
-Blood's nerfed HS can be felt on some fights, but it's not too significant - 2H blood is still the best for fights like Twins where the bosses are tanked together.
-Hinenuitepo prefers Blood 2H right now if it comes down to it, but deep unholy is very fun for the playstyle and pets and aoe.
After a few nights running my new dual dps specs, there aren't really any big surprises.
As you can see here from a full regular TOC 25 clear, I did fairly well. (for the 'fail, fail' cryers, yes, I know I'm not uber - for goodness' sake I had to take the 232 badge shoulders this week just to get the 2-piece T9 bonus). On this run, I did the fights exactly as planned. I was blood for beasts, Jaraxxus, and Twins, and was unholy for faction champs and Anub. I did fairly well on all the fights. My dps was subpar on Twins where my lesser-geared DK counterpart beat me - but that's another blog. In general, though, having the two specs allowed me to better address the mechanics of the fight, and showed on the meters as well as on our success in the raid. My blood spec gave me great burst to get Gormok down quickly, Acidmaw just after one submerge, and Icehowl during stuns. It let me cleave dps onto Mistresses and Twins. My unholy spec let me send my ghoul to harass a healer for faction champs, then a caster - and my 'smart' wandering plague added dps to the melee mess. On Anub we clumped the adds and my diseases, D&D, and blood boil burned them down fast.
Last night I managed to get into a partial guild Onyxia 10. We wiped once due to unfamiliarity with working together with a couple of the pugs, and our rogue foolishly got himself cleaved on the pull. I learned that I had to wait.... wait.....wait... for the tank to get agro on the whelps, because once I spread diseases to the group, then dropped D&D, it only took one little blood boil to not only kill half of them, but to get the rest realllly pissed at me. Even with significant down time (two ranged dps to get Ony to come back down and fight like a woman) where my only dps was Icy Touch and Death Coil on Onyxia in the air, I ended the fight with 6900 dps; not too bad for the 10-man.
All in all, if you can afford to do it, I strongly recommend you do an unholy spec and then either DW frost or 2H Blood, depending on personal preference and raid needs. If you know you won't be needed to tank, this is especially important as it gives you the flexibility to contribute maximally to your raid. There were a couple points at which my group was discussing my going tank, but they made comments that I didn't have a tank spec and managed to find another tank. The key here is that I had my death gate up and ready to go respec for tanking if needed - so I didn't hurt my raid by shirking my possible duty as a tank, but I DID provide maximum utility and dps for the group by having two dps specs at my fingertips.
One side observation; if you're not very very familiar with both of your chosen specs, spend plenty of time on dummies learning the rotation. Maybe farm elite pustulent horrors for a bit. Switching back and forth between specs can make you feel a little dyslexic. I map similar functions on both specs to the same button to make it easier to switch. For example, my 'i' button summons my ghoul in all my specs. My 'j' button is my burst button with dancing rune weapon and gargoyle, respectively. The biggest problem that I have is that I can't find a way to get death coil on the same one; it's my '4' key in blood, but my '5' key in unholy.
All in all though, I'd call it a tremendous success!