Bought this to hang a heavy bag from the steel beams in my basement. It's great! 1. The concept is useful in general because you can roll them to the side and clear space when you're not training. 2. They sell special trolleys marketed for heavy bags but those cost $100-180. The only advantage those give over these is that they have a brake. You don't really need a brake, just hit the bag standing parallel to the beam and it doesn't move very much. 3. The magic happens though when you stand perpendicular to the beam. As you strike the bag will move away so you can practice your footwork while advancing. Also most bags will have a D-ring on the bottom - if you attach a bungee from that D-ring to your back leg you can train while backing up. Doing this has even strengthened my back leg - I'm noticeably quicker circling away and I'm sure it's because I've been training with this trolley.
Grumblebee 
