I've wanted a GPS watch for running for a while, mostly because I had no great way of knowing exactly how far I've been running in Tokyo. Fiddling around on Google Earth gave me some idea, but it's just not the same. When early reviews said the 305 did a great job of keeping satellite lock, even when tree cover or tall buildings block satellite reception, I knew I had to get one.
As it turns out, it's incredibly addictive to track this sort of stuff. It's made even more addictive thanks to MotionBased, a site where you can upload the data from your GPS unit and view your routes projected onto Google Earth maps, graph your heart rate against speed/elevation/time, and other neat tricks.
One thing I hadn't considered, but is also great fun, is that wearing a GPS unit when running adds a "collect them all" angle to running while traveling. It helps keep me running, even when I'm away on business, so I can add a new town/state/country to my collection of GPS pushpins.
The only problem I've had with the Forerunner 305 is that, while it is very good at holding a satellite lock, it needs 20-30 stationary seconds, with a clear view of the sky, before it'll lock on. If you start running before it has a lock, it'll never lock on. Once it has locked on, though, it's solid. So now I just put it down on a flat surface while I stretch and it's locked on by the time I'm finished.
I'm looking forward to taking the Forerunner with me on my bike trip to Hokkaido later this month. Garmin doesn't make an official bike mount for the 305, which is unfortunate, but the Polar makes a bike mounting kit for their heart monitors that works great.