I often use a bag hide, light, the ultimate in portability and they work. I've even had birds land on me.
Drawbacks are that they can be hot (i'm willing to suffer
) and that if it's a real still day you need to keep pretty still also. They are more for an hour here and there than being set up for a long time in my opinion.
Certainly makes a big difference when trying to get closer to your subject.
TobyH