I downloaded HexEdit to take a look at it. It may actually end up being a better hex editor than others I've used, but several UI issues stuck out:

1. I selected a file to view that happened to have a long filename. The editor window's title, and the title under the Window menu were both the munged 31-character version of the filename. Curious, I then tried a file whose name contained Asian kanji and kana characters, and it looks like the filename was displayed with a MacRoman encoding. C'mon people: FSRefs, Unicode, etc...

2. How about giving the user a chance to at least choose the display font size, if not which monospace font to use? That might make it easier to read.

3. "File Comparison Options" has hijacked the standard command-comma used for preferences. Why isn't there just a preferences window that lets you set comparison options, color scheme, etc.?