I tried them last time when I was (i think) level 80'ish and had a horrible time. Slow, difficult, had to use CC, and the loot that dropped was same iLevel as regular dungeon drops that took a quarter the time to kill.

Tried again and this time they were level 100 and pretty much still all heirloom gear, and Mother of Pearl it was it fun. Some "captain" type trash was tough but all other was easy and boss fights went smoothly as long as you do the "keep out of the fire" type basics. And the gear that drops is better than heroic dungeons, the weekly quest also drops a guaranteed iLevel 700 item.

They way I see it, TimeWalking could also be called Multiboxer Training Grounds. You're able to kill quickly, practice your CC, practice your movement, and quickly gear up for raiding. I recommend taking your fresh 100's in over and over and over.