There's a brief article which mentions Plaxo and compares it to Chandler here:

Why PIM’s Are So Compelling?

The Chandler concept is a lot more interesting to me. I'd heard about it a while ago, but it seemed vapourware then.

Looking at the parent website, it seems like they are making significant progress now. They have posted about the acquisition of some new developers from related projects, so there is some significant talent getting to work on this:

Chandler, at the Open Source Applications Foundation