Down south ADSB has extended coverage significantly. Before the ESL radar was fed back into TAAATS you'd be lucky to get identified out of ESL, WSL, BNS etc by F120. It's virtually to the ground there. High level stuff in bound from NZ (GEMAC, KURTT, SNELLY, LOLLY etc) are now ADSB identified 15 minutes prior to the boundary rather than a couple of minutes west. It's making a significant difference to the way controllers are separating (better, easier).
There are areas where surveillance has improved significantly (low level). If you haven't got an ADSB transponder your radar service will be terminated where it is always terminated. VFR pilots are **** scared of talking to controllers and don't take advantage of the services available to them. The reasons for that are beyond my understanding.
So, is there a benefit to the controller? Yes, in some areas a significant benefit (makes life a whole lot easier). Is there a benefit to the airlines and high level traffic, yes, they are more often getting the levels they want due to reduced distances in the application of separation standards. This a moot point however when you're transferring aircraft to an FIR that doesn't have surveillance, it goes back to procedural separation.
Is there a benefit to the GA? It depends if you are using ADSB services. I know one big charter and freight operator LOVES it, he can see exactly where his aircraft are rather than relying on text messages in suss coverage areas or expensive tracking tools. If you're an IFR lighty, yes, increased coverage will give a better traffic picture where there's coverage. If you need help one day, assistance becomes much easier if it's known exactly where you are, same with VFR. How do you quantify what that's worth?
Did there need to be a mandate and did that mandate need to be so out of whack with the rest of the world? Particularly the USA where most of the avionics comes from, where cost will reduce significantly with higher market penetration?...................