Zul'Aman was released with the rest of patch 2.3 two weeks ago today. Due to its place in the progression chain, many guilds were able to burn through it on the first day it was released. Downing Zul'jin is nothing when you're raiding in Hyjal and Black Temple, apparently. Of course, the intended audience for this dungeon seems to be those raiding groups that hadn't seen much action past Karazhan.
My own guild has had some success in ZA. While most of us have at least one or two pieces of tier 5 quality loot, we're not up to Vashj or Kael yet, so ZA still holds some challenges for us. We are still dedicated to pushing through the twenty-five player raids, so we haven't spent a lot of time in ZA, but we've been able to take down three bosses without much difficulty. It's a nice change of pace to face encounters where there is a learning curve, but still a lot of room for error. We haven't had much luck at all with the timed aspect of the dungeon, often with silly mistakes costing us those precious few minutes we needed to rescue the prisoner.