It is a great idea to link exiting contacts to the e-mail exchange with these contacts and contact relevant calendar events, to dos and reminders. It saves the most tedious, error-prone part of deploying an SME CRM: Painful transfer of data to the CRM.
Having said that, BusyContacts is not a full scale CRM solution, but it does exactly what I need. BusyContacts and BusyCal won't win a beauty contest like - say - Fantastical, but it makes me so much more efficient.
For calendar integration in BusyContacts you need to buy BusyCalendar, too, however, at their webstore I just paid US$ 24 for each, BusyCal and BusyContacts bundle.