I'm big on keeping things simple when I run (stopped running with music long ago because I hated having wires on me and stuff on my ears. That said, i've got a 305 and love it! I'll admit I'm a geek. I really like knowing how far I've gone, how fast my pace is, what the elevation changes are, and having every single mile split that I run saved automatically. This really makes it easy to see my personal progress as time goes by. Also very handy for various interval workouts.
Never used the heart rate strap that came with it. Seems like a hassle.
Most definitely. My Garmin is my training partner. I don't know anyone who runs, or bike as much as I do. With the Garmin (I use 405 & 305) I am able to monitor my preformance, and maintain control of my workouts. Heart Rate Training is the most accurate way to train, the GPS just improves the accuracy. I compare routes regularly; if I think that I am running horrible, I just compare the route to another and I can see weather or not I truly ran like garbage.
I have an australian shepherd and he LOVES to run. In fact you have to run them or else they'll find ways to get in trouble when you're not around. I run him in the morning for 4 miles and then my wife will run him in the afternoon. They are great...