I have been a fan of PieSync for some time as it is the only product I know of that can sync both ways (Zapier offers trigger-based events by contrast). With GDPR becoming law on 25th May, PieSync offers an easy way to ensure that all of your contact records are in sync. That means that if you are asked to amend or delete a contact record, you can be sure that you can comply with the request. Of course, PieSync can do much more than this, but this feature could save you a fortune when GDPR becomes law.