Credit cards can be considered convenient, not necessarily good. They are an efficient way to operate within your Spending Plan if they are paid off every month. If you cannot pay off your credit card within the month you receive the statement; it is best to stop using the credit card. You will be servicing debt on the card and all the purchases/sales on the statement have now been bought at a higher price as interest expense is added to each item purchased until you pay the card off. Spend wisely and purchase only what you can pay for.
Interest only loans may fit into your spending plan, but they do not increase your net worth. The more you increase the principal portion of your mortgage, that is paying a principal payment on your loan, the greater your net worth. A simple equation is...you do not own what you owe.