I've never used them.
Not that I don't like them, I've seen people so habitually addicted to them it ruined their behaviour so I try to stay away from them.
Though I do sneak in a sniff from time to time.
I actually used to carry some around everywhere when I went to school in the UK. 

 Sorry, rarely, not never. *
But now I feel like getting one for my desk. 

 Uh oh!! Don't do it! I've only heard of one person who seemed habitually addicted and she was elderly. I go through periods where I'm sniffing them a lot but then stop cold turkey for long periods too.