I think credit cards have caused a lot of headaches for a lot of people. A friend once told me he belived they were "the personification of Satan himself." I had to laugh at the description.
I posted a reply in this thread about the rewards program here:
reward credit card vs regular credit card
As far as having a card for emergencies goes? I would recommend you have 3-6 months of expenses in a very liquid save account, such as a money market. I get a bad feeling when people have credit cards "for emergencies" that can set a whole wave of bad financial happenings into motion!
I personally don't have any credit cards - it's somewhat nice to just deal in cash - feels good. I don't owe anybody anything at any time. It's not "standard" in society, but I love it.