Rated 2 out of 5 stars
It used to be quite good untill they removed reminders for group events.
You can add as many reminders as you wish in google callendar but they don't sync so I would not call it exactly bidirectional either. This is a long lasting problem and I was forced to stop using thunderbird (That happens when you miss meetings or are late for them all the time...) The interface to accept meeting reqeuests should be intergrated in lightning. I get mail invites when they are accepted it should do the same as the plugin acceptation method. Perhaps this is a limitation of lightning as you can not set on what callendar it should accept invites. It is nessesary for decent integration though.
Conclusion: Works great if you are a simple home user and want to organize private things. Completely useless in a bussiness. Needs a lot of work.