* Prices may differ from that shown
I've been using my TomTom Go Live 950 for around a year now so I've had plenty of opportunity to thoroughly test it out.
What sets it apart from other sat navs I've used is TomTom's HD traffic service. The traffic services on other sat navs I've used have tended to be of limited use - I found that information updated erratically and only showed problems on motorways and trunk roads. By contrast, the HD traffic service uses a different system gathered from TomTom devices and anonymous information from the Vodafone network which allows traffic jams to show up even on smaller roads.
In my experience this works well, with the device more often than not identifying standing traffic on most roads pretty accurately. It isn't perfect - it failed to notice a 30 min delay on my main commuting route one morning despite the fact that it normally picks up much shorter delays on the same road every day - but it certainly seems to pick up most problems. The HD traffic information is also available on TomTom's website so you can get a good idea before you buy.
Journey prediction times are usually very accurate, even when using country lines - I have found that some other sat navs assume that any country lane can be taken at 60mph the whole way and have a tendency to include them in the route as a result. By contrast, the TomTom is usually more realistic, only recommending country lanes where they are a genuinely quicker alternative.
It isn't perfect. It can be buggy at times and it could do with more memory storage. It has a tendency to crash and reboot more than it should. Also the HD traffic service seems to disconnect itself too often.
But despite all of that, as a high mileage driver, I would still recommend the device and have found myself using it despite my new company car having an (inferior) inbuilt system.