For me since I have had a floor to ceiling bag, not one of the crappy ebay ones but a proper one. My accuracy and timing has improved a rediculous amount, might not primarily work that but since iv been using it regularly come sparring im able to hit a guy whos trying to bob and weave with ease, time people coming in alot more and pick more accurate and coordinated combinations.
Have a try, theyr not even that expensive to buy and theyr well worth it from my experience so far. From when I first started it I could manage a 3 punch combo - jab cross left hook at full speed. Now im able to land a jab, cross, left hook, cross, uppercut full speed at ease. Generally pitter patter around on it like its nothing wheras before I couldnt do such a thing, and it seems to have transfered right into my sparring and competing.
Picking shots has never been so easy, like they just fall off your shoulders when you throw its great!