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.
The Tom Tom 1 is kicking arse at the moment in Sat Nav reviews in mags such as T3. It's been said their fast becomming the Ipod of the sat nav world. I think it's because their software and interface work well, but higher up the price scale Garmin get just as good results as Tom Tom.
I have (in my wifes car) a Garmin C310 and have also have tomtom 6 on pda, but not a tomtom 1.
Overall the Garmin is much better than the TomTom for ease of use and seems faster in calculating routes and recalculating when taking a wrong turn. Adding POI is a breeze also on the Garmin using the supplied software, but the same can be said for the TomTom