Running E4/X2 combo on my enduro bike with 203 rotors front and rear, and I absolutely love this combination, 2 of my other bikes run E4 front and rear with 203/180 rotors, which is also quite nice. Decided on the E4/X2 for better heat management on the enduro bike, as it's going to be pushed quite a lot harder than my other bikes, so there will be more heat generated, and it seems to work very well in the real world, even for the occassional bike park laps.

E4 with 180 rotor up front isn't powerful enough in my opinion, but as I'm quite light (~75 kg with gear), it's enough when paired with a bigger rotor.