Ranked in these QuestionsQuestion Ranking
Pro Currently free (only while in beta)
If you sign up while Zendesk Inbox is in beta, the product is free for an unlimited number of users.
Pro Easily see what else a user has emailed you about
The "More from..." link lets you see what other correspondence you or your team has had with that user, without having to do a search.
Pro You can @mention your team members in notes
When writing internal team notes, you can @mention a team member to get their attention when they are needed.
Con In beta
It was released in September 2014, but still remains in beta. It may be more unpredictable than a stable release.