SubEthaEdit works well as a source code editor (I don't happen to use its collaborative features so I won't comment on those). It is clean, reliable and has good support. Nice features include:
- Nice, clean find/replace dialog box
- The "function" menu lists classes, functions, etc. nicely indented and with separate icons for classes, functions, etc.
- Always shows you which function or class you are in
- Very adaptable language-specific settings (e.g. for indent and wrap)
- Can highlight what you have changed
- Script menu (but applescripts only)
- Mode menu contains useful language-specific tools (and shows only the tools appropriate for the current language)

However, I do miss some features from other editors:
- No keyboard shortcut editor
- No direct support for scripts written in Python or Perl (you can get there from AppleScript but it's very clumsy)
- No search and replace in multiple files (though I think this is planned)
- No way to diff two files (use Apple's FileMerge instead)
- No "project" support (for managing sets of files)

The undo is a approximately word-at-a-time, which is chunkier than TextMate (one character at a time--ugh!) but still can seem too fine. It could use some way to undo bigger chunks.

Alternative source code editors to consider (though none have SubEthaEdit's collaborative features) include:
- TextWrangler: free, stable and full featured, but has a very clumsy find/replace dialog box.
- Smultron: free and a fairly similar feature set to SubEthaEdit
- TextMate: amazing feature set (though a bit cluttered). I'd probably use it but I find the character-at-a-time undo a show-stopper.

SubEthaEdit editor is a very good editor at a very reasonable price.