Seems that core functionality is OK, but there are some UI issues to be resolved.
Last word from Plaxo user FAQ's is this, as of August 5, 2011:
"Plaxo remains committed to supporting users on the Mac and is aware that our product currently is having some incompatibility issues with the completely redesigned Address Book and Email Apps included with Apple's Lion release. Fortunately, we've found that the most vital part of our product however, syncing of the Mac address book, does work effectively with Lion and your contacts should be updating with all your devices. Our product and engineering team is planning on adding back the UI you're familiar to seeing in the Mac address book and email apps with a new version slated for the Fall of 2011. "
