Definitely not worth it on mage or rogue (if you care about being fast, its xp you won't get at 110 and you will be doing them there probably). Apparently for some classes the xp can be quite good, but sadly I have no xp with paladin.
I would say the heavy RP artifact quest lines are never worth it, basically since via questing you can get every single level on average in 42min, your artifact quest line should give you ~30% of a level if it takes 15min. In my experience many of the quest lines take longer, and are not even 30% xp.