If CF doesn't work, usually it's AMD's lack of driver support that's the problem. Some game engines don't scale well on either SLI/CF, or most likely the developers simply don't care enough to implement the support. That's why multi gpu is reasonable only at point where you don't really care about how much money you are spending on graphics, and are willing to suffer the driver ordeal that is CF and SLI. Sure SLI is better, but I wouldn't call it good by any measure.