The actual equalization is very good.  I listen to a lot of old radio shows, which vary widely in tonal distribution.  

Unfortunately, the app has some quirks.  It never seems to return to the same track on the playlist if the iPhone (in my case, iPod Touch 3G) goes to sleep while it is not actively playing.  This is regardless of the Save position (on/off) in the Settings.  You can imagine how annoying it is to have to search through a playlist of 100 "songs" to manual reselect where you were before.

Also, pressing the middle button on the earphone/microphone changes track rather than toggling pause/play.  

It has no provision for fast forward/rewind using the earphone/microphone switches.