In-car Sat Nav systems usually have huge costs to update maps, and are often very slow in comparison. Of course, they have one advantage - integration and usually a nice big screen.
I used to review SatNavs when they were expensive enough to require a lot of thought. Now people seem to go on price.
I personally think both TomTom and Garmin make good software. Both have good mapping data (Navteq, owned by Nokia, providing Garmin is better set up to do quick map updates and map new areas, but TeleAtlas (owned by TomTom) has improved a lot in recent years).
TomTom, however, has Live HD Traffic which is amazing. If you get a model with integrated SIM card, you also get loads of other benefits like Google Search, which is a gazillion times better than fixed POIs that go out of date before you take the unit out of the box and doesn't need hooking up to your PC to do updates.
But the traffic service is awesome and better than anything else. Why? Because it is getting its information from literally millions of people in real time - including you providing information to other TomTom users. How? By tracking (anonymously) the position of everyone with a Vodafone SIM - including your TomTom LIVE. If a lot of people are detected as moving in a certain area, it can work out that they're clearly driving and on a certain road. There's a great deal of intelligence. This is then used to detect when they slow down/stop. Again, with so many users it can easily filter out the one or two that stopped to park, fill up etc.
The end result is that all roads are covered, although clearly the busier ones will give more accurate data quicker. Nevertheless, I've found it has picked up on even the smallest of jams that other systems wouldn't pick up. The Highways Agency has sensors and there are other older systems, which is how most other devices get traffic data - possibly updated with known road works. Some simply make guesses of traffic, or based on history. TomTom is live!
Mind you, they do have this other thing called IQ Routes, which knows the average speed on a road on certain days/times, which gives more accurate estimated times of arrival.
What a shame TomTom is now finding that most people are using navigation software on their smartphones. They have an app for iPhone, but there are also loads of cheaper (or free) ones that may not be as good, but most people just don't realise that a good navigation package might be worth paying a little more for.
Google Navigation is quite good for Android users (and free) - but download an alternative text-to-speech application, as the system voice is very bad.
SVOX does one and it's
incredibly clear.
Nokia Drive for Windows Phone is very basic, but good and also free (and will soon be updated to add a lot more functionality). CoPilot LIVE is my personal fave for Android, which is what I now use as it's more convenient than my old TomTom (but I have lost the HD Traffic service, which really annoys me).