I'm a long time Thunderbird user, and it does almost everything that I'd expect from a Mail client. When I got my iPhone, I switched to use [http://calendar.google.com Google calendar], because of it's nice interface to export and access calendars (using XML or iCal). [http://www.mozilla.org/projects/calendar/ Lightning 0.7], the Thunderbird Calendar plugin, is now out, so I decided to give it a spin again. Oh man, the new GUI has improved a '''lot''', props to the designers here. On top of that, the [https://addons.mozilla.org/en-US/thunderbird/addon/4631 Google Provider] for Thunderbird also shipped a new update, 0.3, and it works great with the new Lightining version.
Using these new version, I now have a true two way synchronization with my main calendar (Google). I can see upcoming events easily, I get alerts from Thunderbirds of upcoming events. And best of all, when I get one of those pesky Outlook invitations, Thunderbird now shows it nicely formatted (readable), and I can instantly accept or decline the meeting invitation. Since this updates my Google calendar in real-time, my iPhone also gets the calendar events, using the nice iCalendar / iPhone synchronization modules on my Mac.
All in all, I'm very pleased with this setup, take a look at it if you need a professional Calendar solution, and don't want to switch to Outlook and Exchange.