My Dad bought the Garmin C510 for a week before he took it back at exchanged it for the TomTom One.
The hardware of the Garmin is excellent, but the software lets it down. Its very easy and simple to use which is good. Anyone could pick it up and us it, but when you want to starting add waypoints etc and use POI's it starts to struggle.
TomTom is the much better overall package, and still simple to use.