There are many benefits of ADS systems, Mode S, ADS-B, ADS-C, all have different parameters.
NextGen was supposed to go live with ADSB 2 in 2020. This is ADSB-Out only.
At the last conference, about 3 weeks ago, it looked pretty grimm. Bandwirdth is a huge issue. What is to be included in the broadcast is stuck in all kinds of committees and interest groups. The big surprise was that in the FAA budget for 2014/2015, there is no money alloted for ADSB. From what I have heard, I am not sure if the FAA has any budget for 2014/2015
With regards to ADSB-In, there is little chance that any of us will see this implemented. Boeing does not support ADSB-In because of the security/integrity issues. They stated they will only do what is mandated that they do.