I have an 04 525 mxc in my garage right next to my 01 KX5. They are similar in some ways and very different in others. The 525 turns better, supension is alot more plush, feels lighter, and overall is easier to ride.
However, the 525 does not like to be chugged down low in really tight situations. The motor gets choppy and stall prone. The power is smooth and abundant but still lacks the fury of 500 2 stroke power. I still think the KX has a better top speed and gets there alot quicker. For long endurance type races The 525 is a no-brainer. For A$$ kicking and everything else the KX is the weapon of choice.