I've made one before, but I made it for paintball. I did use it on a bow hunt though, and 3 deer came within arms length of me. Only problem is that I either had my bow leaning against the tree because I was eating lunch, or they came up behind me and I didn't know it until I did something that spooked them, which effectively spooked me!
In my opinion, the most important part, if you do decide to make a guillie suit is the head, shoulder, neck area. The human bust is pretty much what gives you up as a human, if you can build up and break up that area, the rest of you doesn't need to be as dense. It also helps to make your suit in the woods you plan to hunt, and occasionally check to see how well it's blending in. And of course, use fresh vegetation from the area you set up in to augment it.