I've been to Ulduar a couple of times now. We killed Flame Leviathan after a few tries on Friday night, and then spent a few hours wiping on XT-002 and Razorscale without looking like killing anything.
FL is a fun fight, and quite easy. Problems generally occur when he aggros a Demolisher, since they are generally not prepared to kite him and die quickly, taking a big chunk of dps with them. I think it won't be long before we are doing this with towers active (hard mode) to increase the number and quality of drops.
Razorscale is an add fight. I hate add fights. The 10 man seems conceptually harder than the 25 man, for the basic reason that you need to do everything with 2 tanks, whereas the 25 raid will have a spare tank if required. The mobs come in waves from the left and right, with the occasional mob spawning in the centre that needs to be zerged down very fast. We got a lot better at it throughout the night, and could reliably get the boss to the ground twice, but did not have the dps to bring him below I think 70%. After thinking it over we were focussing on the medium mobs ahead of the small ones, which was an error. I also need to pay attention to the mobs' chain lightning attack. Otherwise I think this is just practice tbh.
XT-002 we wiped on for several hours, to the point of being depressing. Our best try was around 65%. Where I think we went wrong was not dpsing the boss during the adds phases. The strategies I saw treated the heart phase as a time to kill adds. That's totally wrong. The heart phase is the time to dps the boss as hard as possible, with a few seconds in the transition between phases to mop up any adds left over. I think if we change strategies we might have more success.
We still had two dpsers in our raid doing less than 2k dps. That's not good enough for Ulduar I think.
Overall, other than the first boss the difficulty is way ahead of anything I've tanked before, including Kara. I would also say that it is harder than Zul'Aman, which was a real surprise as I expected it to be around that level of challenge. I was reminded of Blackwing Lair in both the scale of difficulty and the requirement to learn special techniques for killing the bosses.