Ok, so the Timeless Isle is supposed to be a major feature in 5.4, but from what i've seen so far is, You grind tokens for old (496) gear and old (Mogu) charms. Besides that there are some World bosses and a Pet Battle Tournament. Doesn't really sound all that interesting to me. Maybe one day per week for the World bosses, and Pet battles if your interested in them. Farming keys and chests doesn't really seem to make it worthwhile either.
The Timeless Isle is inhabited by all manner of strange and powerful creatures that will test the mettle of solo explorers, but great rewards await the brave and skillful: epic loot, bind-to-account gear, Mogu Runes of Fate, pets, mounts, fun toys, and much more.