I like it. As Eagle_Driver says it's not the most efficient phones around but whether that's an issue depends on what portable you'll be using it with. (which you didn't mention by the way, The_X).
The only problem with the PX100 is the zero isolation. If you're planning to use them in a noisy portable situation they will sound tinny and horrible most of the time, and you'll have to wind the volume up really high to hear anything.