I like the heroic dungeon achievements a lot. They give you something to strive for if you're bored if the instance, or just want an extra challenge. I recently had a great time getting Zombiefest! in what would have otherwise been a pedestrian CoS clear (not enough DPS to try for the timed boss; we were running with some fresh 80s and a Holy paladin DPSing).
However, some of the achievements are currently though to be near-impossible. One that is often talked about is Gotta Go!; typically, it's Anub'arak's burrowing that throws attempts off. Good news to those of you working towards your Red Proto-Drake: the time limit on Gotta Go! is being doubled to four minutes in 3.0.8. Many groups were getting quite close to two minutes before, so this change ought to make it relatively easy.