The death of Xmarks leaves a vaccuum and creates an oportunity for ES.

There is no extension like Xmarks (when it worked well).  It had a user-friendly web UI.  I could expand the different backups and see their content.  I could choose a backup to make it the current syncing bookmark set.  I could remove backups.  I could view the contents of the current set.  Many other options. 

I would say that in terms of features, ES sync/backup is somewhere between the built-in Firefox sync and Xmarks. 

Right now I actually prefer the built-in Firefox sync because it's MUCH faster than ES sync.  For example, after the initial sync, then pressing the sync icon makes it spin for maybe 4-5 seconds tops). 

The one thing I use in ES is the auto-backup feature.  Too bad there is no setting for the time period between backups (I think it's 15 min and it cannot be changed), but it's better than the built-in Firefox backup, which only backs up bookmarks once a day.  So that's the only reason I'm using ES right now. 

This is an opportunity for extensions like ES to fill the gap that Xmarks left, but it would require a lot of work by the team.  Hopefully this thread will provide more ideas for improvement. 

Good luck!