Aye, I'm a big fan. I suspect that in other UK cities, they were commissioned on existing, windy bus routes. Prime example being the introduction of Livingstone's bendy buses on London's tricky network with little or no planning or preparation, and the Merc buses where technically a bit crap.
At least a bit of thought, investment and planning went into our infrastructure and route to operate more like a linear tram route.
I suspect the popularity has taken Translink by surprise. They don't half get bunged at peak time if there's a gap of anymore than 7 or 8 minutes between them and even at that it's a struggle, to the extent that they have complemented it with the old 4X in the form of Urbies at peak times (which has been a success as far as I can tell).