How well the traffic receiver performs (reception wise) is VERY dependant on what area you're in. To start with, you can take a look at
www.realtimetraffic.net for coverage maps. I can tell you that in the New York area the coverage on the map is just shy of grossly overstated. If you happen to dig deep enough to find out what stations broadcast TMC signals in your area, don't assume that reception of their signal on your car radio is any indication that you'll get good reception with a TMC receiver. The TMC signal is broadcast at a lower power level, and has less reach than the "normal" signal.
Regardless of whether TMC looks like it should work for you or not, your other choice (at least with TomTom devices) is TomTom Traffic. If you have a compatible phone and data plan, I would recommend TomTom Traffic over TMC without hesitation - even if TMC reception is excellent in your area.
For my money, TomTom Traffic provides significantly more detail with their data, resulting in better-informed decision making when it comes to re-routing around traffic. Couple that with more broad cell phone coverage (vs spotty TMC coverage in some areas), and to me, the choice is clear.
I originally purchased a 920T, and found TMC performance in my area to be poor. I opted to return the 920T and purchase a 920, and subscribe to TomTom Traffic. Although I only used the 920T for about 2-3 weeks, I used a Garmin with TMC for over one year prior, and the difference is like night and day.