Needs work and testing!! Rated 1 out of 5 stars

Lightning has its own toolbar, which integrates very badly in the Windows version of Thunderbird. Rather than appear as a separate toolbar, it attached itself in the middle of Tbird's standard toolbar, causing some of TB's standard buttons to scroll off the right side (where they were unattainable!).
Opening the View menu, Toolbars... and selecting the Lightning toolbar will turn it off, thank goodness! This is where things really get screwy, when you start turning the Standard and Lightning toolbars off and back on. The behavior will have your screen redrawing and flip-flopping you madly back and forth between standard TB, TB with a badly integrated Lightning toolbar, and Lightning with no TB email in sight.
I do credit Lightning with pointing out a shortcoming in Tbird, which I had never encountered before--and that is, the Standard toolbar does not adapt to an overflow of icons. No drop down list at the right side or any way to scroll to reach icons out of sight.
Because the Lightning toolbar integration issue was so severe, I uninstalled Lightning before using any of the Calendar features.
Fix the toolbar so it appears as a separate toolbar, instead of being appended in the middle of the Standard toolbar, and I'll give Lightning another trial run-through.
And, yes, I did NOT have the toolbars locked when all this maddening behavior was occurring.
I really like Thunderbird, Firefox, and everything Mozilla has ever done. I'm sure you guys will get Lightning right, too--I just don't know how the problems I encountered did not show up in your own testing. I'll watch for Lighting 0.8 and give it another go.

This review is for a previous version of the add-on (0.7).