I first supplemented my cyclometer with a GPS over 5 years ago. When the cyclometer batteries died I didn't bother replacing them and have since removed it from my bike since I much prefer the GPS. Yes, it sometimes loses satellite lock momentarily but not enough to cause problems with the odometer. OTOH, I always have the moving map display in addition to the normal cyclometer functions (speed, av. spd, max. spd., dist., altitude) and it keeps a record of the ride that I store on my PC so I can always print a map of my rides and review data about elevation gains, speeds, times, etc. Very useful when later creating a cue sheet for other club riders.
On organized group rides I find it nice to see exactly where we're riding rather than just following the arrows on the pavement. But it's been most helpful when touring. I use the detailed Garmin MetroGuide maps in my eMap and they include the locations of restaurants, motels, grocery stores, etc. It also lets you enter a street address and shows you where it's located so I can look up bike shops or other stores in a phone book and then ride to the nearest one. Despite the "MetroGuide" name, I've found it to have detailed street information in even the smallest rural towns and to include all country roads.