*Functionality of Streak is incredible it has everything
You can do mass mail check if people saw your email and track when and how it has been opened.
Streak is tightly integrated within Gmail account and creates its own tab where you can have a personal CRM view
while tracking email you can see who has opened it and react fast and surprise a customer or a person you work with. Makes work very comfortable
* Contacts and additional features
Streak has a great way of associating each contact with a specific deal (box) which makes it easy to use gmail for sales. For example, I can easily message all people related to the issue without ever searching for the addresses.
UI needs some work. It does not seem welcoming and even if it has all the features, some user might just get confused.
In terms of functionality It can be a bit faster. Also, I believe, Streak team will work regarding more customization and create more options for users to adjust the plugin to the needs of the company
For my personal projects I use streak to transform my gmail account into a power house where I can handle and track all the work and now lose any valuable piece of information. The sorting of the emails, attachments and history of conversation is all happening for me within the Streak and it does save my time and work.