I have been using Thunderbird for a while now, and I really love it! It does what I want it to and like FireFox it is extensible. I have used Sunbird a bit for calendaring, and I found it to be a great tool, but really wanted that functionality in Thunderbird so after looking around a bit I re-discovered Lightning (a friend of mine had pointed this out before but I forgot), and then I discovered Provider for Google Calendar (found out about it at this well done blog).
Lightning added the basic calendar functions that I needed to Thunderbird and Provider added two-way updating in Thunderbird for my Google calendar!
So now I can check my Gmail and Google calendar very easily within Thunderbird.
In case you need some guidance here is a site with excellent instructions on how to setup Lightning and Provider.