This last weekend I signed up for Plaxo, after watching others use the company's services for the last year or so. Like the author of this article (below), I was suspicious in the beginning but am now satisfied that Plaxo's intentions are honorable enough to let them hold my address book.
Rather than write my own article explaining why I feel better, here's someone else's. His should prove to be more thorough anyway, as he actually met with the founders of the company.
Link: Plaxo Reconsidered.
Related link: Plaxo 3.0 On The Horizon