correct - pay it off by the end of each month and you won't pay interest (you may pay a surcharge on purchases though, particularly on AMEX)
yes, this is the way it works out - I use my AMEX/VISA account as you would with a regular bank account with eftpos and pay it off prior to the end of each month - I now have enough points for a return flight to Europe
any debt is bad when you can't pay it - if you can pay your card off each month then you're sweet - if you can't even pay off the minimum payment, then you're ****ed
set a very low credit limit (less than 1/3 your month's salary) and you'll be fine