There are two TomTom products.
One is the free TMC receiver. Not very reliable, and not exclusive to TomTom either.
The other one is called "Live-Services. It uses date from mobile phone signals and other TomTom users, and can tell you pretty much exactly where traffic jams are, even if they are not broadcast on TMC. It also costs EUR 49 per year.

You can try the live traffic updates with the free Tomtom internet based route planner on their website. Try a routing going from Hamburg to Bonn, or from EC2Y 8HR (Barbican Center) to TW6 1QG (Heathrow Terminal 3), or from Beauvais to Marche de Rungis (ie from north to south of Paris) at various times of the day. The routing changes with traffic conditions, sometimes several times within minutes.