Get the newer Garmin's with the H in the model (legend/vista), those have the more sensitive chipset and will work well between tall buildings or under bridges/tunnels/trees.
I use the neck lanyard wrapped around the handlebars as a safety, if mine would ever come unclipped it would just hang by the lanyard.
Another advantage of the GPS over a dedicated bicycle computer is taking it off the bicycle and on a walking/jogging/hiking trail. I also let mine log to a gpx file on the microsd card and then geotag pictures afterward.