Soocial Highrise Sync
There were two main reasons I went with Highrise for CRM. First, because of its simplicity, and second because of its ability to sync with Soocial. This functionality is advertised on their website yet when you go to actually do the sync, Soocial states that 37 Signals has requested they disable synchronization.
A little investigation points the finger at Soocial. There were bugs in their implementation that resulted in the deletion and recreation of contacts, removing relations and detaching deals and cases from the contacts.