Simple, it works, no bullshit and is very flexible. Free version has all the bells.
Due to the design and deep intergration, can be daunting.
I've used Streak for + 3 years and has always been my go to. I've set it up for clients for sales, project management and my personal inbox.
Support is always willing to help and once you think you have mastered it, there is another hidden feature. (formulas in a CRM anyone?)
Make sure to hold an onboarding session for your team and commit, it will pay off.