It also depends on where you drive. TMC works off sensors on main routes and motorways, so if you mainly drive in the country then you will not get any information relayed to the TMC, however if you use motorways and A roads it might save you some time and identify potential hold ups.
I had it built in to a previous car, along with the sat nav. It sounds like a great idea but isn't always that good in practice.
I was on my way to the centre of Bristol and it asked if I wanted to avoid the traffic. I said 'yes' and it took me on a long detour through the suburbs and made me nearly 2 hours late by the time I got into the centre and worked my way through the same traffic jams from the other side!