I have the RNS 510 in my car, I also have TomTom on my PDA and my phone (And yes I can read a map

, I find Satnav saves on arguments with wife when we she gives dodgy directions).
I personally prefer the overall package of the 510, being able to start the car and its there, no setting up, no time waiting for my PDA to talk to my GPS unit, looks good, large screen, directional arrows just in front of you, can't beat it. It is an expensive unit, but if it is in the car when you buy it then you don't see the cost as such.
The 510 has been invaluable recently altering my route due to traffic problems getting me to where I was going and saving time. I never found TomTom did that for me.
However, I have had a couple of problems with the 510; when I got the car the SD card slot would not recognise my media, the unit was swapped under warranty, and a few months ago the GPS unit died, again swapped out under warranty.
I only use my TomTom when I am abroad, or in wifeys car.