I have one. Like AldbourneRed, I use it for purchases during the month then pay it off in full before interest accrues. The advantage is my money stays in the bank longer and earns interest and I also earn cashback on purchases.
I'm assuming those saying "avoid/don't do it" have got themselves into trouble, for which they only have themselves to blame.
If you're sensible and self-disciplined enough, there's no reason not to have a credit card.
That's all well and good saying that, but if you lose your job and can't pay the money back its not really your fault. I've done everything responsible I called the credit card people and arranged payment plans etc which meant that I can still eat and have a roof over my head.
I do see the advantages of having a credit card and I still have mine albeit frozen in my freezer, but now if I don't have the money to pay for it then I won't buy it. sorry for the rant