It's mostly the refurbished 313s in their new Southern Coastway livery now I think. As long as it keeps the 313s off any route I actually use, im happy.
I do think it's a useful service though. My train in the evening is Victoria to Portsmouth/Southampton splitting at Horsham, and for the people travelling to Bognor (which is alot) its much quicker to get the front 4 fast to Barnham and then change onto the shuttle service (which they will usually hold at Barnham specifically for the fast portion of London train to get in), rather than take the rear 8 which go slow down the Arun Valley eventually terminating at Bognor.
It's a nice service that enables Southampton/Porstmouth to London/Brighton trains to be used to connect at Barnham from Havant/Chichester for instance to get to Littlehampton/Bognor.