I have used both the X2 and X4. Both are built like tanks. I liked the X4 better because I wasn't super fond of the LEDs providing me with accurate readings. But that aside, it's a great mixer/pre-amp.
I don't have trouble recommending either the Wendt or SoundDevices units, but between the MixPre and Wendt X2, I would recommend the X2. Primarily because longevity of battery life, quality of the preamps (it was able to provide the 10 mA of current required for Earthwork microphones without any issues and sounds very clean), and build quality of the X2.
While the X2 has been replaced by the X3, the X2 is still a great unit.
Wayne