Seeing as Cally hasn't put out any sort of celebratory announcement about how much more uptake there's been since serving Birmingham, I don't think it's an especially high amount.
I'm using the Cally next weekend to go from Glasgow down to Birmingham, but honestly it seems like a bit of a waste, as it's scarcely 6 hours worth of sleep when breakfast etc is taken into account. Also, the lack of a lounge at Birmingham Intl is a real mark against.
Next time around, it'll probably be more time-efficient to book the Lowlander, change onto the first northbound service for Birmingham, and arrive at the office after a decent night's sleep. Ideally, there needs to be an option for either a lie-in at Birmingham, or lounge access arranged with AWC.
Surely the more sensible choice would be the 1840 getting into Birmingham at 2307 and going to a hotel.
An hour's trapse to Falkirk (with a post-midnight walk to change station) for the highlander, and on top of that having breakfast on the train, seems somewhat unusual.