I have a shiny new looking one which was given to me 3 Christmasses ago. I've worn it a couple of times.
I have an identical but tatty one about 5 years old but have used it more, been dropped on the ground a few times and well scratched and scraped, rained on, been out in the sun and faded. When it's not too hot I wear it to keep the wife happy, and overall I wear a helmet about half the time I'm riding. I leave the tatty one on the bike when it's parked, never locked. No-one will steal it though. For events where they're compulsory I wear the respectable looking one and do lock it to the bike when left.
The older one is sometimes like Andrew's, rather smelly, but I do put it in a bucket of water every couple of weeks with 2 bricks holding it under and a capful of Milton to kill the bugs, so it's not too bad. No-one will take it away even when it smells respectable though.
I often wish I'd had the idea of making a plastic hat and promoting it as a must-have cycling accessory!