Many bugfixes on the Version 5 seem to make it rather usable now. Please consider that it has not been extensively tested.
For new stuff that lead to Version 5 reaching 1, best see cvs. Some new features include,
- internal 'just links' term theme.
- A LOT of theming help
- Custom term lists in internal theme retain treemenu extended URLs
- Node counts are back.
Too many to mention? See cvs - eight stages and vast swathes of the module re-written. The basic plan was to get back to the usability of Version 2, but include the gains of 3 and 4. Lacks expanding menus, but has the code.