Don't waste your money.
I bought a 510 on special offer from Currys last year. It had the TMC receiver bundled for free. Good job as if I'd paid the asking price for it I would be well p'ed off!
It simply doesn't work anywhere close to reliably. Even on major motorways it sat there saying no signal more often than it was receiving anything. And if it did pick anything up the information could not be relied on in my experience.
Far better is the TomTom Traffic Plus service. This is a subscription service that connects via bluetooth to your mobile phone to download the data. Far better in my experience. You need a mobile with bluetooth (obviously) and a GPRS data connection. I use it moderately and so far haven't troubled the free 4MB download package I have with Orange, it doesn't download huge amounts of data, a few kB at a time.
You can set how often it updates so you can set a compromise between how upto date it is and how much data you download.
In my view it is far better value than the TMC service I endured originally.
http://www.tomtom.com/plus/service.php?ID=3&Lid=1