CFDs should not be used as a buy and hold strategy (which is risky enough doing with shares directly). However, with proper money and risk management and the proper use of stop losses, a medium term strategy is very plausible.
I was using CFDs in the past over a short time period of usually between a couple of days to a couple is weeks, trying to catch small swings with very tight stops. I kept getting wipsawed due to my stops being too tight so had too many small loses for my few bigger wins. And yes I lost some money, almost $5k in one year. I have recently started a more medium term strategy with wider stops trying to catch trending stocks. I have only recently started this strategy and so far have 2 loses and 3 wins.
Just remember that you do get charged a financing fee for holding long position overnight, but for short position you actually get paid the funding fee for overnight positions. My broker charges the official interest rate + 2.5% for long positions and pays the official rate - 2.5% for short positions.
So yes CFDs can be used for the longer term as long as you are implementing proper money and risk management and use stop losses. Just be aware of the implications of using margin and all the costs involved.