I seem to only get about 6-7 years out of the sierra bellows. The failure was sudden and catastrophic when it did too. No signs of dry-rot but the thing shredded on the top side... Kind of like it got stabbed with a knife, not the normal dry rot tear where it's stressed when stored and rips along the crease. Dropped the boat in the water for a weekend. Checked the bilge Friday night it was bone dry, came back Saturday morning to water at deck level then found that tear. The prior set was OE and dad can't remember if he did it when he bought the boat or not... so they either lasted ~18 years or ~26 years.
FYI, when selecting the transom kit I'll use during my restoration, the sierra kit was ~$170 at the time with the gimbal bearing... the merc was ~$279 for the same kit... I picked the merc and hope I get the same ~2x the life from them.
I use the exhaust tube instead of the bellow, it's easier and isn't really much louder. Some boats you may get some more exhaust smell around the transom I've heard said. I like the bubbling sound at idle lol.