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.

Rated 3 out of 5 stars

Diigo is a great service, I don't use much of it's capabilities, but I find it reliable. This toolbar is impractical to work with. I have it hidden and only show it when I need to use something from it, but that's not exacly quick use.
The other think I don't like (hate it, to be mor exact) is editing the Sticky notes.(Sticky notes are why I have installed it.) Say you write some longer note about a section of a web page you try to understand. Now you switch browses tab a to look for aditional info. You get back to then note you were writing...and voilá it's gone!
I you only could find it in Lazarus, but even that is not possible...
To sum it up, does it's job, but contains some flaws and inefficiencies.

This review is for a previous version of the add-on (