I have had a heavy bag and have trained vigorosly with it for more than 4 years now. NineTailedFox, if your going to upgrade to a 100 lbs bag make sure your hands and feet are up to it. and also I do side kicks on my heavy bag, which is around 140lbs and half filled with sand, and I dont have a problem. It seems you just need more practice. I know, from experience, that the side kick is hard to get the dinamics and technique right, but once you get the hang of it youll see that its not the heavy bag its YOU. also DO NOT GET a 150lbs bag that few extra pounds are Allot and youll definitly notice. youll also probobly hurt yourself and be frustrated because the bag seems like its winning because it dosent even give or wobble when you hit it thus your doing your power harm because you wont be able to "Dirll through" your target thus leaving out resistance part of the workout getting a stinky workout. id say, work up to it like I did. (I stared with 100lbs bag)