One thing to be aware of is that it is not as accurate in cities as it is outside them, for reasons that are obvious when you think about how HD Traffic works. Nobody is going to be walking down a motorway (or indeed most A roads) so any mobile phones detected are highly likely to be inside moving vehicles. But in a city the opposite is likely to be true. As a result, TomTom has to rely on information from RDS-TMC and its own devices in cities, which reduces the sample size.
The other issue we have discovered in our travels is not really something any traffic system could really be expected to deal with. On many journeys, the decision to use one route rather than another often commits you to certain roads long before you actually use them. For example, when heading into London, you can take any number of roads inwards.
From the West, you could turn off the M25 onto the M3, M4, M40, or even the M1. The problem is that once you have decided which major artery to use, this limits your choice of roads later in your journey. If these roads are clear when you make the decision, you will be prompted to take that route. But then later on if they become jammed you have no choice but to sit in traffic.
In other words, your judgement and experience will sometimes be called upon before gambling on one route over another. Although TomTom's other clever bit of traffic-related tech, IQ Routes, attempts to use historical traffic information to counteract this problem, we still found there were occasions where enduring one traffic jam would have been preferable to a later one which hadn't developed at the time we changed routes, even if this was a far less frequent occurrence than with RDS-TMC.
Overall, HD Traffic has made our journeys far more serene. Believers in the value of market competition will also be pleased to hear that TomTom doesn't have this technology to itself, either. NAVTEQ has something similar in the pipeline, thanks to its purchase of T-Systems Traffic in November 2008. This hasn't arrived in any devices just yet. But the future looks bright for more informative traffic updates - which is great news for the UK's increasingly congested roads.