Good points Merv. One big problem at the moment is the audio alert on TCAS is patented to TCAS, and is not permitted to be used on ADS-B. There will need to be some legal negotiations to allow any ADS-B 'in' unit to talk like TCAS does.
At the moment, there is no real advantage at all. The airlines will still rely on TCAS for their Traffic and Resolution advisories.
For Airservices, once ADS-B is mandated it means controllers will know where everyone is. Those who believe in conspiracy theories say this could lead to a billing system, or more people being pinged for VCOs, but who knows. I don't know how it could work for controllers anyway. There will be a lot of VFR traffic showing, it could be more difficult for them.
I don't believe Airservices too much about the difficulty of maintaining SSR stations. In NZ they have just serviced their SSRs to last for a long time, and if the bl**dy kiwis can do it, I'm sure we could. (No offence to any kiwis on this forum).
Mike is correct about the standards. The problem is that at the moment we are looking at 1090ES, the US is using 1090ES for high level, and UAT for low level (GA). UAT gives extra benefits like real time weather displays, but we can't use UAT here because I think the frequency is not available. The disadvantage though is that UAT still requires an additional transponder, and 1090ES does not.
However the 1090ES is not the standard in Europe either. VDL-4 is another system being trialled somewhere over there.
Some people have been saying we shouldn't rush ahead because there is no real advantage yet. Maybe we should use TCAS, and wait to see what the world standard ends up being - sort of like Beta or VHS. Then hopefully all the problems will be ironed out, maybe there will be an audio alert, and maybe companies like Garmin will mass produce units for GA at a reasonable price.
Ken there is no transponder requirement at all for GAAP. There are aircraft flying there which do not have a transponder fitted at all. For those that do, the usual procedure (certainly here in Sydney) is to leave the transponder on STBY for circuits and turn it on if you plan to depart. The reason for this is that the tower controllers don't have secondary radar, so they cannot see transponder squawks anyway, but they can see out of the window. Transponder squawks do show on the radar controllers' screens, and they don't have any control over circuit traffic at distant airports (they are not on the same frequency as the traffic, and there are already controllers there who can see out). It just leads to extra clutter on the screen.