I use the Heavy bag once in the while. It gives me a different feel when I kick it and it helps me develop power along with speed. Learning to develop a good punch and some open hand techniques has a different feel on the big bag but I enjoy those work outs because i defiantly can feel my mistakes.
I like em for 30 and 60 seconds reps. of hitting and delievery as many kicks or punches as I can. It is mainly a way of cardio training for me these days, but like others have said it does give a different feel when hitting them.
I have an XXL Wavemaster in my back yard that I hit often. I boxed for so long that I separated my hands from my kicking combinations. So now when I kick I feel akward. I need to work on it to re-integrate them together again.
I prefer one or more Centry standalone bags. I put 200 lb of sand in the bottom for support. If I want animation I use moving van rollers (for moving refigerators.)
I don't like heavy bags unless I need to work on pure power. The moving bags that give some are much better on one's joints.
Oh, and one of those bags is a "Bob". He wears a t-shirt to!
I used to do more bagwork when I practiced at the Y, because they had one. The new facility I attend for now doesn't have one and it's killing me.
I focused on back side kicks, sliding side kicks, and roundhouse kicks. Broke the chains holding it three times. The fitness personnel just loved me!