I use thunderbird because the interface is better for bulk managing email than the webapps from either gmail (my previous provider, and my employer's current provider) or fastmail (my current provider). For work, the real threaded view is a big improvement over the gmail UI collapsing it into linear conversations. Calendar integration is there and is mostly "fine". I won't say it's competitive with google calendar as it exists today, but my needs of calendar are less than my needs of mail.