I was just about to ask that!!
When ive used it ive noticed the lo end becomes less tight even with a little amount added. And the high end sounds really grainy and peircing when i add like any more than 1/3s.
I dont really like the thing, but i heard this mix a few months back that the guy had used pretty rubbish equipment and said he used the Sonic Maiximizer on almost all his single tracks and it was one of the best mixes ive heard in the MP3 mixing clinic yet!