Modr 'S' will be a big benefit to ATC for aircraft in CAS. From about 2005 we will be able to see some FMS info from the auto pilot (enhanced mode'S'). Mode S altitude will come down to the ground in 25ft increments. All this can be used to help safety in CAS. I am not an expert, but I cannot see what benefit Mode S can be to GA outside CAS, unless it will be used for TCAS.