TomTom is the best, NavMan and Sony would come second. I have sold TomTom for more or less two years now and they have caused the least trouble and have been by far the most easiest, reliable and most functional GPS we have ever stocked. Go for the 510, its a great all round GPS or i noticed in todays paper the TomTom One for £200, thats a Bargain. I have noticed the NavMans going for the same price, but i have found them not to be as feature rich as the TomTom. The TomTom cant be beat on ease of use, nor can it be beat on finding addresses quicker.
TomTom is great for speed cameras too, it tells me if i am entering a hot spot for police activity, it tells me where the temp cameras are on the motorway, and most important it lists the fixed cameras. Gives 200m or so warning, some cases on the motorway a mile.
I have the 510, i couldnt warrant the extra Money on the 710 or 910 as i dont go to Europe in my car, nor do i go to the states in my car. The 510 can get me to the main places in Europe though if i ever needed it.