I used a standard webbed waist belt for years until I read a study that Deer & Deep Hunting magazineon treestand falls and harness systems. Luckily I never had a slip where I had to test the belt system, or I may not be here writing this!
Last year I purchased my first really good treestand, a summit and it came with a free 5-point harness system, and amountain climber strengthclimbing rope, that uses a nifty knot system called a "prusic knot", that clips to my harness and allows it to slide easily up and down but grip when any weight is placed on it. It's so easy to hook up and use, I use it all the time.
Now climbing with my harness on, and properly attached to the tree gives me the piece of mind to enjoy the experience of being in the woods.
As a result of the article, I also carrymy GPS and cell phone in my pockets at all time in the event of a disaster I can phone in my position to someone.