I compared it A-B to my Rat and in the Classic mode it is nearly identical. The Magus retains more bass overall compared to the Rat, not sure if that's a good or bad thing, I guess it will show in a band context. The original Rat sounds a bit more honky overall, but that could be variance from Rat to Rat even, due to component variance.
All the modes sound good. The gain knob has a much more even taper than the original Rat that goes from 0 to 7 in the first few degrees of rotation and then stays between 8-10 for the whole remaining rotation, so that's an improvement over the original. The switch doesn't have a tactile click when turning on/off. It is smaller that you would imagine from the photo, great for pedalboard real estate, plus all jacks are on the upper side.
One big downside is that the switch engages not when you press it but when you release it. This can make it very hard to activate the pedal exactly when needed in a song. But if it is always on it is not a problem.