It does it's job. It's nothing too mind blowing with Sonic beating up the bag with a basic combo. I did like how the bag seemed to change its "face expression" when hit due to the reaction to Sonic Black's punches. This is the fourth animation I have seen of yours so far. Is there anything that you would like to add to them to spice up the repetitive pattern?
Perhaps some music to help with tone? Added audio for voice acting? Or perhaps going for a bit longer storyline? Even if it's just by 10 seconds. Maybe you can change the angle we see the animation (although I imagine that's difficult). You could possibly add audio for Sonic screaming with a kiai as he strikes the bag to make it look more like training. Even just having his mouth open as he strikes would help. Of course I understand that other flash media with Sonic doesn't have that. I'm just throwing out ideas.
Do you have any other story lines planned so your work doesn't get repetitive? Not just for the viewers but for you as well so we don't both get burnt out with the same short animation? I am 100% sure this takes a lot of hard work and dedication just to make a couple of seconds. But striving for some improvement can help. I mean, this is your sixth animation. I am wondering when you will want to change it up a bit.