had a Garmin 405, switched to the Nike + GPS watch as didn't need all the Garmin functionality. Loved it and did everything i needed at the time..
But now in marathon training, i wanted some more features again, so have just acquired the 620 with HR/Run sensor, and i love it... really easy to use, syncs over wifi, great screen and so much data to pour over!
had a garmin 405cx and sold a year ago for the nike gps watch.
Garmin - took ages to find a satellite (a lot longer than Nike on average), battery charge seemed unreliable - unit would occasionally die on me with no battery though indicated 100% when starting. There is a lot of stats but i didnt use most of them. Can sync the garmin to more of the online running sites (fetch etc), with the 405, never really took the touchscreen.
Nike - imho looks better and can wear as everyday watch, charges through simple USB (and syncs), quick to pick up signal, Nike has better website from a 'social' aspect and for recording run info, easy to use/understand/set up..
am pleased i got the nike (and recently added the HRM) - there are however some things i miss from the Garmin, such as the virtual pacer and alarms on speed/pace, and has more advanced sessions you can plan in advance and then perform these sessions on the run
with the advent of parkruns, many younger kids now love running... my son (8) plays a lot of football, swimming and now loves parkruns... does them when we can and is already running just over 23mins.. and there are a number of others within a few mins of him.
I love seeing them all run together, going fast/slow etc... and he loves it..
Not sure how he'll progress, but i need to watch it... won't be too many years before his old man gets very concerned!