I think most Mac users (okay, i'm generalising) have become used to their software looking good. There are already many editors for OS X that have simple, elegant interfaces and that follow Mac interface guidelines. e.g. Textmate, Coda, Espresso, even Xcode and MacVim to a point.
In my opinion Ultraedit for Mac is ugly and looks like what it is, a port of some Windows software. I'm sure it's a fantastically powerful editor and this might seem shallow, but this is enough to stop me from using it. If I'm going to stare at an interface all day it needs to be visually pleasing.