Rated 3 out of 5 stars
Lightning is a good add-on, but with some attention to basic functionality, it could be a FANTASTIC add-on.
One suggestion is to include native synchronization with Google Calendar. I currently use Provider and it works well for synchronizing my calendars.
It also needs to synchronize natively with Google Tasks. This basic function is sorely missing in both Lightning and Provider.
Email clients have been around for quite a while and such basic synchronization functionality is currently the norm rather than the exception.
Keep up the good work and please consider the above suggestions