I personally think the standard Carista is poor value too.
I did the setup stuff in the free period, and that's it. It'd be cheaper in the long run to get something like an iCwrsoft V2.0 that can read all systems as well as trigger regens, configure new components, calibrate new senors etc, than pay £xyz every month for Carista.
It's far from being the end of the world but with the benefit of hindsight I wish I hadn't bothered.