I finally had a chance to run Kelly Flats for the first time 1-2 weeks ago. It starts with a steep climb of Heart Attack Hill and ends with a fun obstacle called the Chutes. Great drive through Poudre Canyon and some impressive views from the top. Being my first run, I chose the left side of Heart Attack Hill. I think this was a good choice for me, as apparently a gentleman in a group a ways behind us rolled on the right side.
Trail Info: http://www.traildamage.com/trails/index.php?id=37