I've been using the BusyMac suite of BusyCal and BusyContacts for years now and feel they deserve an upate. These programs work seamlessly with iCloud and offer features that simply don't exist in Apple's native apps. (I'm not sure why BusyContacts isn't coming up on the App Store but I highly recommend it as a basic CRM app.)
In addition to these being well-developed apps, I want to mention the excellent tech support offered by the developer. I usually get a response by email within 24 hours and they have never failed to figure out the problem and provide a solution on the rare occasions when I've had an issue. 5 stars all the way!