We've been using ZOHO One for almost 2 years - there is a module for almost every business function and we love that they are all in one place.

When we started, we were mostly focused on CRM - lead tracking, conversions, task management, customer management, etc.  The CRM module can be customized in a million different ways.  

We built ours out by having a weekly team meeting - so that the opinions of Sales, Customer Service, Accounting, and Management could all be heard.

We can schedule tasks for ourselves (or each other), maintain important contact data (phone, email, etc), and monitor our pipeline (of leads and customers) in one place.  And since it was a team effort, we all use it faithfully - company wide - to keep up with all the important aspects of our business.  Highly recommend it.   

There are other modules within ZOHO One that could be used for Activity Tracking ("Desk", "Marketing Hub", etc) that will integrate with the CRM module, including calls, emails, etc.  Good luck!