IMO vs Flock

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.

They have apps for Windows, Mac, Chrome, iOS, Android and web interface. And all the chat sessions are always on sync.

It is half the price of Slack and full of nice features like polls, assign to do's, etc.

Unlike its rivals, it not only allows you to receive events, but also allows you to push content to it and receive various user action events.

Moving a team or organization to flock does not require any setup from the company's IT team. At max, what might be required would be to whitelist their domain in case the organization's IT team has gotten external domains blocked. You can simply download Flock and invite others in your organization by using their email ids in order to start collaborating. That's all there is in the name of setup.

Most of its features are free, even video conferences. But also they have a Pro version with unlimited chat history, Active Directory integration among other things.

There is no limit on the number of groups or of group members. Similarly, within a team you can have unlimited members. There is no limit of number of teams too.

Flock is offering an app that is faster than Slack.

For every team, the permissions and restrictions can easily be set over the participating members and the conversations they are having.

Posting messages is more fun with them in addition to emojis.

There are multiple options like slash commands, message buttons, chat buttons, attachment picker etc., where the integrations can surface inside the native app. This greatly eases the discoverability of a integration, making it really easy for the team members to find and use the integrations.