I use one: it’s easier to fit/remove than a wedge of anything. We used to use an old pillow but removing it and refitting it so there were no gaps was a huge pain in the arse. Bear in mind that dust and crap fall down the chimney, so when you remove whatever you’ve shoved up there it gets very messy, and whilst it might sound fairly easy just to wedge something back up there I found it surprisingly difficult to do with success.
The chimney balloon is very much easier. I inflate it using an old camelbak tube and it expands to fill the gap no matter what the shape of the chimney is at that point, and when I deflate it (sucking the air out of it) all the crap falls into the fireplace and not all over the floor.
Beware though, I’m on my second. The first one I bought lasted less than 1 minute, as I tried to fit it the morning after having a fire, which despite me checking still had a glowing ember in the grate. The slightest touch of the ember on the balloon bag shrivelled and holed the balloon rendering it useless (this by the way is a useful feature, as if a fire was lit with the balloon in place it shrivels to nothing and you don’t get gassed!)
HTH
Ben