The strongest points of Mushkin's HP-580 AP power supply are its extremely competitive price and the well thought out modularity feature. All cables are sleeved, the two PCI-Express power cables even have some additional circuitry inside them to improve power quality.
The RailFusion feature is designed to help you in the rare situations when you are overloading a single 12V rail (more than 240 W). This could happen when you connect a large number of devices using Y-cables to split the power. While power delivery and the system were stable at all times, even when testing on our CrossFire X1900 XTX setup, it is evident that a 580W PSU has its limits. So you should maybe not use it to power two overclocked Radeon HD 2900 XTs, but for almost everything else the Mushkin HP-580 AP should be a fine choice.