I use them for the rewards, mainly for things that I am going to buy anyway. I really like the different rewards catagories every quarter with Discover Card, things like gas and groceries. Why not get 5% cash back on things I buy anyway? Now American Express is doing the same and I enrolled the other day. My Citibank card is a Dividend card so I get cash back there too.
If they start charging interest from the time of purchase I won't use them anymore. No savings there.
I don't carry much balance on cards, normally pay them off each month. I do have one at 0% until October that I am paying a little bit each month. Why pay it off when I can keep that money in my high yield savings account and earn interest on it until September? The only other thing I owe besides mortgage is my auto loan, which is paid of in THREE MORE PAYMENTS!! Yoo Hoo!