Live traffic, if I have read everything correctly, requires maps and more to be paired (via bluetooth) to a smart phone. It is free but will most likely use some of our monthly data allowance (but only a tiny portion).
As I forgot to turn bluetooth on for my first proper drive with M+M, cannot confirm this though..
It's free for 4 weeks, I think, then something like £20 or £25 one-off payment (to Garmin store) to continue.
It uses bluetooth tethering which only worked when I added the tethering add-on to my Three account (in fact I got a text from Three saying something like "you're trying to use tethering and you don't have it enabled on your account").
In terms of the amount of data used, it appears to be very small.