Not sure about the Coast Road services, but from what I've seen of the X21 extension so far, there only a small number of people travelling to/from West Auckland.
From what I've been told, concessionary passengers are starting to use it over the Arriva 6, even using it to only travel from West to Tindale to catch the Arriva X1 to Darlington.

I wonder if the numbers will start to pick up, although one of the main issues is that while the first X21 from West gets to Newcastle at ~8:30am, the the last X21 to West leaves Newcastle at 5:10pm, which means people working a 9-5 can't use it to get back, so are likely to still use Arriva. If they made the last bus down an hour later, I think they'd see more people using it (assuming there are people that actually travel to Newcastle, and not just connect to other Arriva services at Durham)