Good idea, but foreign characters not supported.
I have several Chinese contacts, and Cobook doesnt use the "phonetic name" fields whereas Address Book does. Those names end up at the very bottom of the list, and the only way to find them is to type the actual characters.
Unfortunately, that makes this otherwise nice application mostly unuseable for me. I wish someone had written a review mentioning this.
Also, among the quirks, there is a fonctionnality to merge contact information, but only by using drag and drop, if you want to merge the first with the last of 2000 contacts, you have to scroll through the whole list.
pcairic about Cobook, v1.0