IMO vs TeamSpeak 3

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.

Allows for complex administrative structures, with lots of roles and many other features related to server administration.

It has an advanced system to manage channel/server permissions.

Available on Windows, Linux and OS X allows for a user to choose what best OS suits them and not have to worry if their game chat software will work.

TeamSpeak allows for both voice and text chat. this adds flexibility, and can clarify information if somebody has a very poor connection, or a very thick accent. It also allows players without a mic to communicate with the other players.

Among most MMOs (massive multiplayer online games such as Guild Wars 2, or World of Warcraft) Teamspeak is widely used so it's likely other players already use it and won't need to download a new program. There are also many knowledge and help articles available online.

As long as one is not using TeamSpeak for commercial services the app is free to use. This is great as most users will not be using it for commercial purposes (as long as you aren't making money off it you don't need to pay for it) such as gaming.

This allows for very granular organization.

You can tune everyone's sound output levels to meet your own personal preference in inbound volume. So to your ears nobody is too loud or too quiet.

Under "Edit Channel" --> "Advanced" --> theres a toggle button called "Voice Data Encrypted". Everything but file transfers is encrypted.

A wide range of plugins are available for those that want extra functionality, ranging from soundboards and overlays, to peripheral and application hooks.

You can host your own server.

Signing up for a MyTeamSpeak account will allow you to synchronize your bookmarks, identities, hotkeys, whisper lists and channel subscriptions across all devices. MyTeamSpeak features will include addons synchronization, addons update management, and more.