Pretty great at this time Rated 5 out of 5 stars
Awesome features: You can use it to bookmark the site both online in the Diigo service and in the firefox bookmarks; definable keyshortcuts; You can always download a backup of whole your collection in a zip from the Diigo site.
Some small drawbacks: Sometimes the popup menu above a highlight doesn't appear, reloading the page usually helps. Switching the windows or tabs sometimes closes an unsaved note, which can drive you mad.
Sumary: When you get used to it, it totally makes your browsing workflow.