I have had my Garmin Forerunner 305 for about a year and a half now. Before buying it, I used to run with the Nike + attachment for my ipod. However, I stopped using it after I started having corrosion problems with my ipod due to condensation build-up inside of the Nike + armband.
I enjoy using my Garmin as it provides me with a more accurate indication of my pace and mileage. I think this has certainly been beneficial in my marathon training as it has allowed me to more accurately maintain the pace I should be running my tempo runs and speed work at in order to have a chance at hitting my goal on race day.
If there is a downside to running with something like a Garmin it is that it is easy to become obsessed with your data. I am trying to improve but on occasion I still find myself checking my pace and distance more often than I need to. When I catch myself doing this I force myself to stop looking at it for a set period, not looking at it until I reach a certain point along my route etc., and just allow myself to run based on how I feel.
Overall, I am glad that I made the decision to purchase a Garmin as I feel that it has benefitted my results.