IMO vs Missive

Although IMO has a comparable, and in some cases superior, feature set when compared to the competition its key selling point is its interface.

From LifeHacker when it won "Best IM Client":

Signing on, navigating chats by tapping the tabs at the bottom, searching for buddies, and virtually any other operation is easy to find and use. The app, itself, is very quick. Even sending a voice IM, if you don't feel like typing, is responsive and sends quickly. Everything imo does it does well, but it's excellent navigation really makes it top notch.

IMO also has a fully functional website that you can use for chat while on your desktop or laptop. This really puts IMO ahead of the competition as you can use the same system on all your devices so you don't have to deal with overlapping notifications etc. (Note: IM+ also has this feature)

It should also be noted that it has the highest rating in the App Store due to both the UI and its stability.

Tabbed chats make it easy to switch between conversations

Both your buddy lists and chat histories are searchable.

Concurrent sessions allow you to be logged in on your desktop while also being logged in on your phone.

In Missive you can live edit an email draft with your teammates, the draft composer acts exactly like a Google doc. Each team member has a different color cursor.

The email section allows for consuming all of your email accounts in it, utilizing shared addresses (help@acme.com), personal work inbox (philippe@acme.com) and personal inbox (phil@gmail.com).

It’s important because the collaborative part of Missive becomes useful when users consume all of their emails within the app. You not only want to collaborate around emails sent to your company help@address, but also around the really important emails you most likely receive at your personal company address, like that really important email you received from a potential high profile partner.

Missive is also a team chat app. When using Missive you can ditch your email client, chat app and help desk.

Merge all email accounts in one unified inbox.

You can create multiple tasks per conversation all assignable to a single or a group of persons.

Nicely written software. Always works as you'd expect, beautifully designed, a classy piece of software.

Supports multiple organizations with one user login.

Know exactly if and when the recipients read your important email.

Supports Gmail shortcuts.