Have only been using it for a few days, but so far I have found it to be an absolute pleasure to use. Lifts Firefox functionality to a whole new level.

Navigate quickly and easily from one group to another, move tabs from group to group easily, and at any time reload all tabs in a group with one simple click. Easy to customize, plus a manual/auto backup feature.

It's a 10 out of 10 from me. Allow 30 minutes to familiarize yourself with it, and set it up the way you prefer. Recommended.