Traffic reporting
In practice, ground stations are sending lots of traffic up and are not directional, so you can "see" a lot of traffic both directly from Out-equipped aircraft and indirectly from the ground station feeds to other aircraft. But if you do not have Out you cannot guarantee that your immediate airspace will be included in the traffic feed from the ground stations. Bob is correct that few airports are line-of-sight to a ground station but most are line-of-sight by the time you are about 1000 AGL.
The FAA has a cool KML file you can download to display the ADS-B ground station coverage by altitude. I'll try to find it and add it to this post. Here it is:
http://www.faa.gov/nextgen/equipadsb/airspace/
Distance filtering of traffic typically takes place at the display end - not the ADS-B receiver. For example, GPSMAP x96 devices only draw up to 8 targets that are within 7 miles and +/- roughly 3,000 ft - targets outside that range are ignored. Other displays, like ForeFlight, can display dozens of targets at long ranges.
As more aircraft add ADS-B Out, more traffic will be reported by the ground stations.
Some nice side effects of adding ADS-B out: You can use your Flight Aware account (you do have one, don't you?) to track your plane and even send text messages to loved ones when you take off and land. My wife is currently getting pelted with texts whenever I fly the -6. This can be a problem if you do pattern work...
Since you mentioned the ADS600-EXP, there is now an iPhone/iPad app to configure and test it more easily and the EXP is a very cost-effective solution for legacy installations. We installed an ADS600-EXP in a D180/GTX-327 equipped RV-12 and most of the wiring hook-up can be done at the back of the transponder.
A side-effect of installing that box is that I ended up writing the App for NavWorx. It was just accepted by the App Store. Sorry - shameless plug.