I think when it's done well, it's effective, at least for out-of-city routes like Coaster and Regency (and higher frequency routes like the Metro routes they used to have). It's a useful visual cue because people will recognise it as an established, easy-to-use service to Brighton rather than relying on having to look at the number on a generically liveried bus. I know when they started running the Regency route to Hailsham, at least a couple of people mentioned the new route to me having recognised the buses running around, but like you say, they've never really fulfilled this potential because of the mix of liveries and odd workings waters down this impact.
I think once they have replaced all the existing buses with the new liveried 400MMCs, along with the corresponding branding on the site, timetables etc, it will definitely make a difference and make bus travel more attractive to people.