Ok, Plaxo has just gotten perfect with the 3.0 version of their product. Now I can use Google Calendar and Gmail as endpoints along with Thunderbird and iCal, Address Book, and Mail. This means that 2-way syncing between Google Calendar and my other calendar sources is automatic through the Plaxo conduit -- works the same for Outlook, I'm sure.
I really like the new Plaxo web interface as well -- once I turned off all of the Flickr stuff and changed it to a black skin, I've got an amazingly cool web interface that is a single source for all of my calendar items, tasks, and contacts.
I've been a Plaxo user since the service started and the continuing improvements like those found in 3.0 keep my a loyal user. There's actually been enough of an improvement in the service that I'm trying the premium service for 30 days -- the big hook for me is that they do secure back-ups of all of my information with version control for restorations. I'm also going to play around with the LinkedIn integration to see how that works.