We had 6 months to play around in Naxx before Ulduar came out, a lot of that was time spent leveling and gearing for raiding. My guild run cleared Naxx a couple weeks after Ulduar came out running 3 hours a week without any extensions.
Ulduar was out for less than four months before ToC was introduced. Looking back on this the timing could not have been worse. Here we had a dungeon that rivaled and even exceeding the quality of ICC in many respects given less time as the current raid tier than ToC was. My 3 hour a week guild run got as far as Hodir and was starting to discuss raid extensions when ToC came out.
I feel my raid could have gotten a lot more enjoyment out of Ulduar had blizzard held off on TOC another month at least, preferably 2 or 3. Once something new came out with better drops for the same effort the old content was all but abandoned. Between badges and the new ToC 5-man there wasn't any reason to enter Naxx or Uluar anymore.
ToC was excellent for a 3 hour run though, all four months of it until ICC came out. Once ICC came out we were clearing ToC so fast we had time left to try out the first couple bosses in ICC. Eventually moving to ICC full time. That was the first transition since Naxx to Ulduar that felt right.
Now we've had over 9 months in ICC, more than twice that of any other raid content. It's getting old fast and I've still not even seen the Lich King fight yet. I think that little movie spoiler in Dalaran removed like 70% of my desire to kill him. It would have been nice if a large chunk of that time in ICC could have been spent in Ulduar.
Then of course we had our 1 boss dragon instances to fill extra time, do something different and such. Those are still a really great way to fill some time after extending an ICC instance to a second week. I'm hoping that Cataclysm dungeons will be about 1.5-2 hours long on farm so that the transition from one tier to the next can be a gradual one.
It is a sad thought that there are many people who have been raiding since forever and have a Lich King kill, but not a Yogg kill. I think in addition to Blizzard waiting until it is ready to release they should also wait until we are ready. There are always going to be those who want more right away, but I think I represent the middle ground when I say we got too much too soon and then nothing.