Love the Green Belt! I've now lived on the very north end as well as the south end, and it's such a perfect trail for bike riding, running, or just a long walk.
On a personal, quirky note, I love that there are lots of "landmarks" I can use while running. You know, "Just make it to that bench and you can take a walk break" or "OK we're gonna sprint until the end of that overpass!" Sounds hokey but it makes me run longer and harder! Big points for a small time runner.
There is definitely lots of traffic all throughout the trail, so stay aware of what's around you (don't blast the iPod up to 11) and stay on the right side of the path so you aren't run over by bikes (or roller blading moms with strollers).
The only negative I've ever found is that after a rain (or after the sprinklers come on), some parts can get muddy and smelly. But those spots are few and far between. I feel super luck to live right on the Green Belt. A desert gem for sure!