When comparing IMO vs iMessage, the Slant community recommends iMessage for most people. In the question“What are the best messaging apps for iOS?” iMessage is ranked 5th while IMO is ranked 20th.
Ranked in these QuestionsQuestion Ranking
Pros
Pro Video call support
Pro Beautiful User Interface
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.
Pro Easy to switch between conversations with tabs
Tabbed chats make it easy to switch between conversations
Pro Lists and chat histories are searchable
Both your buddy lists and chat histories are searchable.
Pro Accounts can be linked so you don't have to sign in to several accounts
Pro Push notifications
Pro Concurrent sessions
Concurrent sessions allow you to be logged in on your desktop while also being logged in on your phone.
Pro Chat groups
Pro Secure end-to-end encryption
Pro Cross-platform within Apple ecosystem
iMessage is available on iPhone, iPod, iPad, and Mac.
Pro Supports voice messages
Short voice recordings/messages can be quickly sent, and messages can be easily dictated into written messages as well.
Pro Free and unlimited messages over Wi-Fi and data
Messages clearly indicates whether messages are being sent over iMessage (blue) or SMS (green) where messaging rates apply. iMessages do not count as SMS messages with your carrier, instead they use a negligible amount of data to send over Wi-Fi and cellular data networks.
Pro Clear indicators for message delivery and receipt
The app indicates when messages have been sent, when they have been read, and when someone is typing.
Pro Comes pre-installed
No need to download or sign up for a new service.
Pro Seamlesss switching between SMS messages
Pro Friends don't need to "sign up"
You can just message them using their existing phone number or Apple ID and they will get your message.

Pro Clean UI
UI is minimalistic, and clear. And there is a plethora of stuff on the left of text box if you wanna use them.
Pro Apps and games for iMessage are available
For example, you can play games like pool or you can show someone a destination to go to.
Cons
Con No support for third-party instant messaging networks
On March 3, 2014, IMO will start discontinuing support for all third-party instant messaging networks.

Con Not cross-platform
iMessage is not available on anything but Apple devices.
Con Can't send separate texts to individuals who share a Contact record
iMessage is an utter disaster. It can't send separate texts to anyone who is on a shared Contact record but with separate mobile phone numbers (e.g., "Paul and Angie Smith"). It merges ALL threads that ever went to ANYONE on that shared Contact record. Want to text just the husband to set up a surprise party for the wife? Not possible. iMessage will ONLY add that new text to the EXISTING thread you had with the wife. Duh! iMessage is pitifully incompetent.
Con Blocking Spam Messages takes 3 steps more than is needed
You have to open the message, select the sender, open info on the sender, then block sender. The default Android message app, select the message, block sender. it automatically deletes the message and blocks the sender.
Con iCloud sync of message is not stable
Sometimes, the messages on different devices with the same ID will be different, the syncing function work poorly.
Con Slow and unreliable
