I think HexFiend has the potential to become a must-have Mac app, but there are a few features I would personally like to see before adopting this as my killer-app must-have hex editor:
a) "Jump to Offset" should be able to handle hex offsets. Also, when I jump to an offset, it would be nice if the position was highlighted, not just the line.
b) Font sizes / choices should be persistent across relaunches.
c) It would be nice to be able to visualize data as more than int, float, hex and MacRoman. I'm thinking other useful visualizations would include binary, octal, decimal, signed/unsigned integers of various widths, and various character set representations (Latin-1, UTF-8 sequences, UTF-16, etc...).
d) A killer feature would be the ability to view raw offsets on the disk (disk editor functionality). For instance, I'd like to be able to view the first 512 bytes of my hard drive (the Master Boot Record), which is represented nowhere as a file object. Probably a good idea to view this as read-only! :D
e) Another killer feature would be the ability to specify templates, so that I could annotate and describe segments of data in a file. ResEdit had TMPL resources to do this, which were simple and elegant ways of visualizing data structures.
All in all, I like where Hex Fiend is headed, and can't wait to see future versions of this project!